#!/usr/local/bin/perl

use strict;
use Data::Dumper;

my $s = '<img src="http://www.blah.com?one=[KEY]&two=[KEY]"/>';
my $value = '13.95';

my $q_key = quotemeta('[KEY]');
my $q_value = quotemeta($value);

$s =~ s/$q_key/$q_value/g;
#$s =~ s/\\././g;

print Data::Dumper->Dump([$s], [qw/s/]);