Data::Dumper is adding the extra backslash:

$ perl -le'
use warnings;
use strict;
use Data::Dumper;

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

my $q_key = quotemeta( q{[KEY]} );
print $q_key;
my $q_value = quotemeta( $value );
print $q_value;

$s =~ s/$q_key/$q_value/g;
print $s;

print Data::Dumper->Dump( [ $s ], [ qw/s/ ] );
'
<img src="http://www.blah.com?one=[KEY]&two=[KEY]"/>
13.95
\[KEY\]
13\.95
<img src="http://www.blah.com?one=13\.95&two=13\.95"/>
$s = '<img src="http://www.blah.com?one=13\\.95&two=13\\.95"/>';
[download]

And as others have said, you don't need to quotemeta the string half of the substitution operator.