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

##</code><code>##

my @need=sort qw|ABC GHI MNO|; 
my $flat=join "",@need; 
my $re="\\b" . join( "|",@need)."\\b";

if ($flat eq join( "", sort grep{m/$re/}@FILES)){
   say "Found!"
}