Marais:

I'd guess that the decode_entities routine is directly manipulating the @_ array containing the subroutine parameters. If it does that and makes any changes to the string, then it can do what you're describing.

Here's a cheesy example of what I'm talking about:

$ cat destructive_sub.pl
use warnings;
use strict;

my $param = 'Now is the time';

print "$param\n";
do_it($param);
print "$param\n";

sub do_it {
    $_[0] =~ s/([it])/uc($1)/ge;
}

krevulax:~ [roboticus]
$ perl destructive_sub.pl
Now is the time
Now Is The TIme
[download]

Notice how the second print statement gives a slightly different result.

...roboticus

When your only tool is a hammer, all problems look like your thumb.