My recommendation is to use perl 5.8.0 or more recent and look at perldoc Encode, perldoc open, and perldoc -f open. If tr doesn't work because you have the characters encoded in two bytes, you can do

$s = decode_utf8($s);

That will convert the string into the internal representation where characters are characters and you don't have to worry about how many bytes they need for encoding.

I think the problem is not on the string (I'm using perl5.8.5, because 5.8.0 had some bugs in RedHat), but on the tr operator itself.

The first attemp works like this:

perl -e '$_="áéíóú";tr/áéíóú/aeiou/;print'
aeaoauauau
[download]

It seems that "á" is treated as two characters, maybe "´" and "a", and each one get one different matching char ( "a" and "e").

BTW, encode and decode functions return values that make me think that the string is well formed, and that is tr// who's making wrong things. Am I too lost?

If you have utf8 encoded strings in your program file, you need to use the utf8 pragma (see perldoc utf8).

use utf8;

$s = 'holáéíóúon';
$s =~ tr/áéíóú/aeiou/;
print $s;
# prints holaeiouon
[download]

The code above may show the double characters explicitly since perlmonks.org is served as ISO-8859-1.