Maybe this is what you mean — i.e. replace (for example) 'cat' with 'dog', but only if 'Cat' is capitalized, the replacement word should also be capitalized, otherwise not (?)

#!/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
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This could also be extended to handle all-uppercase words, too, or in theory any cOMbinatION, in which case the source and replacement word would have to have the same length, though.