use warnings;
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]');

$s =~ s/$q_key/$value/g;

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

__END__

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