Actually, there are situations where one would like to read file names from STDIN -- e.g.:

ls *.log | show-the-alarms
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in which case, it'll be handy to have the file name(s) included in the output (I haven't tested this):

while ( $file = <>) {
  chomp $file;
  my $status = "open file";
  open( MYFILE, $file ) or next;
  $status = 0;
  my $last = "";
  while (<MYFILE>) {
     if(/ALARM:/) {
        print "$file:", $. - 1, ": $last";
        print "$file:$.: $_"; 
        $status++;
     }
     $last = $_;
  }
  close MYFILE;
} continue {
  print "Done with $file : found $status error(s)\n";
}
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That might not be what the questioner had in mind, but it's a reasonable approach.