use warnings;

use Data::Dumper;

sub mysub {
        warn Dumper \@_;  # globals still set
        die 'single word expected' if (my $name = shift) !~ /^\w+$/;
        warn Dumper \@_;  # you "lost" the globals (due to matching /^
+\w+$/)
        die 'single word expected' if (my $value = shift) !~ /^\w+$/;
}
$str = "abc def";
$str =~ /(\w+)\s+(\w+)/;
mysub($1, $2);

__END__

$VAR1 = [
          'abc',
          'def'
        ];
$VAR1 = [
          undef
        ];
Use of uninitialized value $value in pattern match (m//) at ./t.pl lin
+e 12.
single word expected at ./t.pl line 12.
[download]

my ($name, $value) = ($str =~ /(\w+)\s+(\w+)/);
mysub($name, $value);
[download]

  use 5.010;

  use strict;
  use warnings;
  my($foo, $bar) = (1, 2);

  sub baz {
      say shift;
      $bar = 3;
      say shift;
  }

  baz $foo, $bar;

  __END__
  1
  3
[download]

perlvar says "read-only and dynamically scoped"

They are. mysub does indeed back them up on entry and restore them on exit.

$ perl -E'
   sub f { say $1; "b"=~/(b)/; say $1; }
   "a"=~/(a)/;
   say $1; f(); say $1;
'
a
a
b
a
[download]

The problem is that $_[0] and $_[1] are aliased to $1 and $2, so changing $1 and $2 changes $_[0] and $_[1].

$ perl -E'
   sub f { say $_[0]; "b"=~/(b)/; say $_[0]; }
   "a"=~/(a)/; f($1);
'
a
b
[download]

Three solutions:

• /.../;
  mysub("$1", "$2");
  sub mysub {
     /.../;
     shift;
     shift;
  }
  [download]

• my ($x, $y) =~ /.../;
  mysub($x, $y);
  sub mysub {
     /.../;
     shift;
     shift;
  }
  [download]

• /.../;
  mysub($1, $2);
  sub mysub {
     my ($x, $y) = @_;
     /.../;
     $x;
     $y;
  }
  [download]

Try it like this: \(&mysub(<$1> <$2>));

#!/usr/bin/perl -slw

use strict;
use re 'debug';

sub mysub {
    die 'single word expected' if (my $name = shift @_)  =~ /^\w+$/;
    die 'single word expected' if (my $value = shift @_) =~ /^\w+$/;
}

my $str = 'abc def';
$str =~ /(\w+)\s+(\w+)/;
print "\(&mysub(<$1> <$2>))";
[download]

Try it like this: \(&mysub(<$1> <$2>));

What does that do?

---

Improve your skills with Modern Perl: the free book.

It's a dressed up version of this:

print "'$1' '$2'";