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