use warnings;
use strict;

# The terms are to be sorted in this order:
# SM (summer)
# AU (autumn)
# WI (winter)
# SP (spring)


sub by_term {
  my($t1, $t2)=@_;
  if ($t1 eq 'SM') {
    return $t2 eq 'SM' ? 0 : -1
  }
  if ($t1 eq 'AU') {
    return 1 if $t2 eq 'SM';
    return $t2 eq 'AU' ? 0 : -1
  }
  if ($t1 eq 'WI') {
    return 1 if $t2 =~ /SM|AU/;
    return $t2 eq 'WI' ? 0 : -1
  }
  if ($t1 eq 'SP') {
    return $t2 eq 'SP' ? 0 : 1
  }
}

my @a = @b = qw(SM AU WI SP);


# by_term() returns the right integer for each possible comparison

for my $a (@a) {
  for my $b (@b) {
    warn "$a - $b" . by_term($a, $b);
  }
}


# however, the test suite yields only the input array!

my @try = (
	   [qw(WI AU SM SP)],
	   [qw(SP AU SM WI)],
	   [qw(AU WI SP SM)]
	   );

for my $try (@try) {
  my @sort = sort by_term @$try;
  warn "@sort";
}