my %subs = (
   foo => 'bar',
   bar => 'baz',
);

##</code><code>##

s/(foo|bar)/$subs{$1}/g

##</code><code>##

s/(Xaa11|Xaa1)/$subs{$1}/g

##</code><code>##

my $pat = join '|', map quotemeta, sort { length($b) <=> length($a) } keys(%subs);
s/($pat)/$subs{$1}/g;

##</code><code>##

my $pat = join '|', map quotemeta, keys(%subs);
s/\b($pat)\b/$subs{$1}/g;