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+package feature;
+
+our $VERSION = '1.13';
+
+# (feature name) => (internal name, used in %^H)
+my %feature = (
+ switch => 'feature_switch',
+ say => "feature_say",
+ state => "feature_state",
+);
+
+# NB. the latest bundle must be loaded by the -E switch (see toke.c)
+
+my %feature_bundle = (
+ "5.10" => [qw(switch say state)],
+### "5.11" => [qw(switch say state)],
+);
+
+# special case
+$feature_bundle{"5.9.5"} = $feature_bundle{"5.10"};
+
+# TODO:
+# - think about versioned features (use feature switch => 2)
+
+=head1 NAME
+
+feature - Perl pragma to enable new syntactic features
+
+=head1 SYNOPSIS
+
+ use feature qw(switch say);
+ given ($foo) {
+ when (1) { say "\$foo == 1" }
+ when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
+ when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
+ when ($_ > 100) { say "\$foo > 100" }
+ default { say "None of the above" }
+ }
+
+ use feature ':5.10'; # loads all features available in perl 5.10
+
+=head1 DESCRIPTION
+
+It is usually impossible to add new syntax to Perl without breaking
+some existing programs. This pragma provides a way to minimize that
+risk. New syntactic constructs can be enabled by C<use feature 'foo'>,
+and will be parsed only when the appropriate feature pragma is in
+scope.
+
+=head2 Lexical effect
+
+Like other pragmas (C<use strict>, for example), features have a lexical
+effect. C<use feature qw(foo)> will only make the feature "foo" available
+from that point to the end of the enclosing block.
+
+ {
+ use feature 'say';
+ say "say is available here";
+ }
+ print "But not here.\n";
+
+=head2 C<no feature>
+
+Features can also be turned off by using C<no feature "foo">. This too
+has lexical effect.
+
+ use feature 'say';
+ say "say is available here";
+ {
+ no feature 'say';
+ print "But not here.\n";
+ }
+ say "Yet it is here.";
+
+C<no feature> with no features specified will turn off all features.
+
+=head2 The 'switch' feature
+
+C<use feature 'switch'> tells the compiler to enable the Perl 6
+given/when construct.
+
+See L<perlsyn/"Switch statements"> for details.
+
+=head2 The 'say' feature
+
+C<use feature 'say'> tells the compiler to enable the Perl 6
+C<say> function.
+
+See L<perlfunc/say> for details.
+
+=head2 the 'state' feature
+
+C<use feature 'state'> tells the compiler to enable C<state>
+variables.
+
+See L<perlsub/"Persistent Private Variables"> for details.
+
+=head1 FEATURE BUNDLES
+
+It's possible to load a whole slew of features in one go, using
+a I<feature bundle>. The name of a feature bundle is prefixed with
+a colon, to distinguish it from an actual feature. At present, the
+only feature bundle is C<use feature ":5.10"> which is equivalent
+to C<use feature qw(switch say state)>.
+
+Specifying sub-versions such as the C<0> in C<5.10.0> in feature bundles has
+no effect: feature bundles are guaranteed to be the same for all sub-versions.
+
+=head1 IMPLICIT LOADING
+
+There are two ways to load the C<feature> pragma implicitly :
+
+=over 4
+
+=item *
+
+By using the C<-E> switch on the command-line instead of C<-e>. It enables
+all available features in the main compilation unit (that is, the one-liner.)
+
+=item *
+
+By requiring explicitly a minimal Perl version number for your program, with
+the C<use VERSION> construct, and when the version is higher than or equal to
+5.10.0. That is,
+
+ use 5.10.0;
+
+will do an implicit
+
+ use feature ':5.10';
+
+and so on. Note how the trailing sub-version is automatically stripped from the
+version.
+
+But to avoid portability warnings (see L<perlfunc/use>), you may prefer:
+
+ use 5.010;
+
+with the same effect.
+
+=back
+
+=cut
+
+sub import {
+ my $class = shift;
+ if (@_ == 0) {
+ croak("No features specified");
+ }
+ while (@_) {
+ my $name = shift(@_);
+ if (substr($name, 0, 1) eq ":") {
+ my $v = substr($name, 1);
+ if (!exists $feature_bundle{$v}) {
+ $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
+ if (!exists $feature_bundle{$v}) {
+ unknown_feature_bundle(substr($name, 1));
+ }
+ }
+ unshift @_, @{$feature_bundle{$v}};
+ next;
+ }
+ if (!exists $feature{$name}) {
+ unknown_feature($name);
+ }
+ $^H{$feature{$name}} = 1;
+ }
+}
+
+sub unimport {
+ my $class = shift;
+
+ # A bare C<no feature> should disable *all* features
+ if (!@_) {
+ delete @^H{ values(%feature) };
+ return;
+ }
+
+ while (@_) {
+ my $name = shift;
+ if (substr($name, 0, 1) eq ":") {
+ my $v = substr($name, 1);
+ if (!exists $feature_bundle{$v}) {
+ $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
+ if (!exists $feature_bundle{$v}) {
+ unknown_feature_bundle(substr($name, 1));
+ }
+ }
+ unshift @_, @{$feature_bundle{$v}};
+ next;
+ }
+ if (!exists($feature{$name})) {
+ unknown_feature($name);
+ }
+ else {
+ delete $^H{$feature{$name}};
+ }
+ }
+}
+
+sub unknown_feature {
+ my $feature = shift;
+ croak(sprintf('Feature "%s" is not supported by Perl %vd',
+ $feature, $^V));
+}
+
+sub unknown_feature_bundle {
+ my $feature = shift;
+ croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
+ $feature, $^V));
+}
+
+sub croak {
+ require Carp;
+ Carp::croak(@_);
+}
+
+1;