You might want to change

foreach (@files) {
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to

foreach my $file (@files) {
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Then you could replace

    $outputfile = "STEP2_" . $_;
    open(OUTFILE, ">" , $outputfile) or die $!;
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with

    $outputfile = "STEP2_" . $file;
    open(OUTFILE, ">" , $outputfile) or die "opening $outputfile $!";
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The value of $_ is almost certainly used for more than the name of the file being processed. These changes will guarantee the filename is retained, and associate the actual filename with the error.

Revise: The foreach was corrected, thanks to moritz kindly pointing out my gaffe.