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if ((my $x=grep {/ABC|GHI|MNO/}@FILES) == 3){
  print qq|Found them all\n|
}else{
  print qq|found only $x\n|
}

my @need=sort qw|ABC GHI MNO|; 
my $flat=join "",@need; 
...
if ($flat eq join( "", sort grep{m/$re/}@FILES)){
   say "Found!"
}