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use strict;
use warnings;
...
    $str =~ s/^R(?:([12])|(3))?$/${1}text$2/;
    $str;
}

sub munge {
    my $str = shift;
...
             {(defined $1 ? $1 : '') . 'text' . (defined $2 ? $2 : '')
+}e;
    $str;
}