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#!/usr/bin/perl

...
require HTTP::Request;
require HTTP::Response;
use HTTP::Request::Common;

foreach (@ARGV)
{
...
    die "Usage: $0 inputfile outdir\n";
    }
}

    die "Usage: $0 inputfile outdir\n" unless scalar @ARGV == 2;
  #I prefer "scalar @LIST", some prefer $#LIST, 
  #but remember the difference
   my ($inputfile, $outdir) = @ARGV;

        if     ($filenum =~ /\d\d\d\d/) {$filenum = $filenum; }
        elsif    ($filenum =~ /\d\d\d/) {$filenum = "0$filenum"; }
        elsif    ($filenum =~ /\d\d/) {$filenum = "00$filenum"; }
        else     {$filenum = "000$filenum"; }

   $filenum = sprintf("%04d", $filenum);