#!/usr/bin/perl
use warnings;
use strict;

my %hash = ();

$hash{a} = "123";
$hash{b} = "456";
$hash{c} = "789";

my $key = "c";

if ( defined($hash{$key}) && hash{$key} =~ m/\w+/ ) {
    $hash{$key} = "Numbers: " . $hash{$key};
}

print $hash{$key}, "\n";