use strict;
use warnings;
use Data::Dump qw/pp/;

my @key_array = 'a' .. 'z';
my @set_array = ( ['a', 'b'],  ['e','f'], ['f','g'] );

my (%neighbors,%group,%mark);

# collect all neighbors per vertex
for my $pair (@set_array) {
  for my $vert (@$pair){
    push @{$neighbors{$vert}},@$pair;
  }
}

#pp \%neighbors;


# mark all connected vertices with same group-id
my $group=1;
for my $vert (@key_array) {
  mark($vert,$group++);
}

#pp "group",\%group,"mark",\%mark;

# sort result by size and alphabetically
pp sort {@$b<=>@$a or $a->[0] cmp $b->[0]} values %group;

sub mark {
  # recursivly (deep) mark neighbors
  my ($vert,$group)=@_;
  return if $mark{$vert};
  
  $mark{$vert}=$group;
  push @{$group{$group}},$vert;

  for my $neighbor (@{$neighbors{$vert}}) {
    mark($neighbor,$group)
  }
}