Use a capturing group and negative lookahead:
/(Eggs|Legs) and (?!\1)(?:Eggs|Legs)/
I.e. the first and the last word can be Eggs or Legs, but capture the first word (using (...)), and then make sure and isn't followed by that word (using negative lookahead ((?!...)) for whatever the first word was (\1)).
See it in action here.
This will capture both words surrounding and and will print them both unless the first captured word is the same as the second one:
use strict;
use warnings;
open my $in, '<', 'in.txt';
while(<$in>){
chomp;
my ($match1, $match2) = /(\w+) and (\w+)/;
print "$match1\t$match2\n" unless $match1 eq $match2;
}
The regex you can only think of is not correct: it misses some parentheses. It in fact matches any string containing Eggs, Legs and Eggs, or Legs.
The correct form would be
/(Eggs|Legs) and (Eggs|Legs)/
To make it work, just check that $1 ne $2.
print if /(Eggs|Legs) and (Eggs|Legs)/ and $1 ne $2;
(Eggs|Legs) and (Eggs|Legs).