use Date::Calc qw(Delta_DHMS);

my $string1 = '12:34:26';
my $string2 = '14:48:52';

my @time1 = split /:/, $string1;
my @time2 = split /:/, $string2;
my @fakeday = (2000,1,1);  # Let's assume you don't want different days

print join(':', (Delta_DHMS(@fakeday,@time1,
                            @fakeday,@time2))[1..3] );
__END__