#!/usr/bin/perl -wl
use strict;

my %repl = (
   cat => 'dog',
   foo => 'bar',
);

sub repl {
    my $word = shift;
    my $lookup = lc $word;
    return $word unless exists $repl{$lookup}; # nothing to replace
    my $is_up = $word =~ /^[[:upper:]]/;       # determine caps style
    my $repl = $repl{$lookup};
    $repl = ucfirst $repl if $is_up;           # apply caps style
    return $repl;
}

my $s = 'cat Cat foo Foo Blah blub';

$s =~ s/\b(\w+)\b/repl($1)/eg;

print $s;   # dog Dog bar Bar Blah blub