• open (INFILE, "$ARGV[1]") or die "$ARGV[1] cannot be openned : $!";
  while ($source_file =~ /(\d)\s+(\d)\s+(\d)\s+(\w+)/)
  [download]

  Maybe (probably) in your head there's a connection between INFILE and $source_file, but not in your script.
  For Perl, they're unrelated…

• if ($ARGV[0] eq "-a")
  [download]

  How do you know your $ARGV[0] really equals "-a"?
  What happens if it doesn't?
  I recommend to add an else block with a print "whatever\n", at least while you're still experimenting.
• Many of the suggestions to your questions include using strict and warnings. Although this seems to impede or slowing down one's development, I recommend it, too:
  • Yes, the time it takes before your script "runs", will be longer
  • The time until your script works correctly, will be shorter
  One of strict's messages is confusing for beginners:

  Global symbol "$x" requires explicit package name

  should read

  Do you really want "$x" to be a global symbol? Better declare it with my

use strict;
use warnings;
use diagnostics;
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noted. Thank you very much for your support.

Also, if your on Windows like me, I open($file, '<', shift) or die "$!"; and then immediately binmode($file);.

Commands or parameters or filenames added to the command line when calling the script get put into an array called @ARGV and when you call shift it increments $ARGV[0] to $ARGV[1] to $ARGV[2] and so on for each shift used.

So, if you used C:\path\to\script\perl my_script.pl file_1.txt outfile.txt then you could use shift again to open() (use three arg open) and instead of printing it to the console window, you can write the output to $outFile.

use strict;
use warnings;

my %hash;

open (my $inFile,  '<', shift) or die "$!";
open (my $outFile, '>', shift) or die "$!";

binmode($inFile);
binmode($outFile);

while (<$inFile> =~ /(\d)\s+(\d)\s+(\d)\s+(\w+)/){
push @{$hash{$4}}, $1, $2, $3;
}

print $outFile "@{$hash{$_}}[0..2] $_\n" for sort keys %hash;
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Usage: C:\path\to\script\perl my_script.pl inFile.txt outFile.txt
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Also, please note this also removes one space from each column per row. As long as that does not corrupt your data set it should be fine. It actually may save you some hard drive space. :)

EDITED: fixed typo in matching patterns, thanks haukex

EDITED: changed and made obvious that the individual needs to make absolutely certain that this does not corrupt anything in their data set.

EDITED: had to add a new paragraph so my second EDIT looked ok.

It is better to open the file using open INFILE, "<", $ARGV[1] or, even better, open $inFILE_HANDLE, "<", $ARGV[1]. It is not causing you a problem now because open() opens for reading (called the "mode") without explicitly setting mode (e.g. reading: "<")

There is a problem reading the file. First you open a file using open(), for that you get a fileHANDLE, e.g. the INFILE or $inFILE_HANDLE. Then you loop reading from the FILE-HANDLE using the diamond operator while(<$inFILE_HANDLE>){ print $_ } and then you close the file(HANDLE): close $inFILE_HANDLE;. You can't read any file contents from a variable which just stores the fileNAME.

In the case your input is not unique wrt the fourth column you will miss input, all similar column-four lines will go to the hash keyed on column-four overriding any previous line with same column-four key. One solution is not to use a hash but an array of arrays (these are formed in exactly the same way as you do now with the regex) and sort works on arrays.

See Re^3: Sort alphabetically from file and Re^3: Sort alphabetically from file, they already gave you hints for file open/read problems.

> In the case your input is not unique wrt the fourth column you will miss input,

That's my biggest problem with this hash approach! Thanks for pointing it out. :)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}