tr/// does not use regular expressions so your title has thrown everyone off.

tr/// only handles compile-time values so you have to use eval to get tr/// to deal with variable data.

If you are going to use the tr/// more than once, then for efficiency, you should do something like:

sub generateTr {
    my( $from, $to )= @_;
    my $sub= eval <<END;
        sub {
            my( \$copy )= \@_;
            \$copy =~ tr/\Q$from\E/\Q$to\E/;
            return \$copy;
        }
END
    die "$@" if $@;
    return $sub;
}

my $xlate= generateTr( "abc", "def" );
my $c= $xlate->( "abc leppard" );
# $c is now "def leppdrd"
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        - tye (but my friends call me "Tye")