tr/// is strictly for character-to-character transliteration. You want substitution.

%change = qw( green yellow  blue red );
my $rx =
  join '|',  # alternation
  map { quotemeta }  # make it a safe string
  sort { length($b) <=> length($a) }
    # so that 'broken' matches before 'broke'
  keys %change;

$str =~ s/($rx)/$change{$1}/g;
[download]

This is a common idiom.

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;