#!/usr/bin/perl -w
use strict;

my $a = "foo";
my $b = "bar";
my $c = undef;

if ( $a =~ m/aa/ ) {

}
elsif ( $a =~ m/bb/ ) {

}
elsif ( $b =~ m/aa/ ) {

}
elsif ( $c =~ m/aa/ ) {

} else {

}