In mysub(), aliases to the global variables $1 and $2 are held in @_, but those globals don't stay set as you expected for very "long":

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.
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Save off global variables, or avoid using them:

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