my %terms = ( foo => 'FOO', bar => 'BAR' );
my $re = join '|', map{quotemeta} sort {length($b)<=>length($a)}keys %terms;
$re = qr/($re)/i;

while(<DATA>){
    s/$re/$terms{lc($1)}/g;
    print;
}
__DATA__
foo is a word
Bar ba blacksheep