#!/usr/bin/perl -w
use warnings;
use strict;

my %translate = (
    "string a" => 1,
    "string b" => 2,
    "string c" => 3,
    true       => 1,
    false      => 0,
);

my $match = "\Q" . join ("\E|\Q", keys %translate) . "\E";

while (<DATA>) {
    s/($match)/$translate{lc $1}/ige;
    print;
}

__DATA__
what is the best way to translate "TRUE" and "FALSE" to "1" or "0" respectively?
or "string a", "String B", "String C" to "1-3" respectively.