Good general advice... in this case, however, every match will succeed -- the test is required to spot the key matching itself. For the paranoid one could write:

foreach my $k (@k) {
  next unless $s =~ m/(.)$k(.)/ ;
  next if (($1 eq "\0") && ($2 eq "\0")) ;
  delete $h{$k} ;
} ;
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but the next unless is redundant.