my @terms = qw( foo bar baz fool foolish );
my $re = join '|', map{quotemeta} sort {length($b)<=>length($a)}@terms;
$re = qr/($re)/;

while(<DATA>){
    my @matches = $_ =~ m/$re/g;
  next unless @matches;
    print "Got @matches\n"
}

__DATA__
Only a foobar foolish fool
skips the sort by length
Without this foo you look foolish
and may match short elements!

##</code><code>##

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