use strict; 
use warnings; 
use 5.010; 
 
my $str = "9999.txt"; 
my $pattern = shift; 
my $repl = shift; 
 
if ($str =~ $pattern) {#now $1 has a value 
 
    my $repl = eval qq{"$repl"}; #double double quotes are needed 
 
    $str =~ s/$pattern/$repl/; 
} 
 
say $str; 
 
--output:-- 
$perl 2perl.pl '(\d{4})' '00$1' 
009999.txt

$str =~ s/$pattern/qq{"$repl"}/ee; 
#or 
$str =~ s/$pattern/eval qq{"$repl"}/e;