I think that /g could prevent the recursion problem, so i have two way that would work :

# version 1
## problem, does not skip comments
undef($/);
my $text = <fp>;
foreach my $pat (keys %my_replacement) {
    $text =~ s/$pay/$my_replacement{$pat}/g;
}
print $text;
[download]

... and ...

while (<fp>) {
    if (!m/^\s*\/\//) {
        # skip comments
        next;
    }
    $text .= $_;
}

foreach my $pat (keys %my_replacement) {
    $text =~ s/$pay/$my_replacement{$pat}/g;
}
print $text;
[download]

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may the foo be with you