If the replacement rules can be formulated in a look up table just use a hash:

my %lookup = ( bar => 'thing1', foo => 'thing2');
$_ = "this has bar and foo\n";
s/(bar|foo)/$lookup{$1}/g;
print;                     #=> this has thing1 and thing2
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If the replacement requires computation, then use the /e modifier which evaluates the replacement side of the s/// operator as Perl code:

sub foo { return reverse uc shift }
$_ = "this has bar and foo\n";
s/(bar|foo)/foo($1)/eg;
print;                     #=> this has RAB and OOF
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