use strict;
use warnings;

my $pattern = qr{(?<!\\)(%[^\s]+)};


my @strings = (
    '%.02f foo',
    'foo %.02f',
    '%f.02f',
    'foo %0.02f bar',
    '\%f.02f',
    'foo \%.02f',
    '\%.02f foo',
    'foo \%.02f bar',
);

my $key = join q(), map chr rand 255, 0 .. 2048;

foreach my $string (@strings) {
    my $pos = 0;
    my @parts = split /$pattern/, $string;
    my $c;
    my @enc;
    foreach my $part (@parts) {
        my $enc = $part;
        if (!($c++ % 2)) {
            my $len = length $part;
            $enc = $enc ^ substr $key, $pos, $len;
            $pos += $len;
        }
        push @enc, $enc;
    }
    my $estring = join q{}, @enc;
    print "String: ($string) => Encoded: ($estring)\n";
}