In addition to the excellent link provided by Tommy above, I would direct your attention to the CGI::Upload module which is designed to facilitate the uploading of files via HTTP. This module incorporates methods for verifying the content and MIME-type of the uploaded file - For example, the following code incorporates a check to ensure that the uploaded file is a PDF file:

#!/usr/bin/perl -Tw

use CGI;
use CGI::Carp qw/ fatalsToBrowser /;
use CGI::Upload;
use IO::File;

use strict;

my $upload = CGI::Upload->new;

if ( defined $upload->file_name('userfile') ) {

    unless ( $upload->mime_type('userfile') eq 'application/pdf' ) {

        #   Error handling to be incorporated here which will be trigg
+ered if the
        #   uploaded file is _not_ a PDF file
        
        die "Uploaded file is not a PDF document file";
    }

    #   Open the output binary file stream

    my $output = IO::File->new( $upload->file_name('userfile') );
    die $! unless defined $output;
    binmode( $output );

    #   Read the file data from the uploaded file stream and write thi
+s data to the 
    #   output file stream

    my $buffer = '';
    my $input = $upload->file_handle('userfile');
    while ( read( $input, $buffer, 1024 ) ) {
        print $output $input;
    }

    $output->close;
    chmod 0777, $upload->file_name('userfile');

}
[download]

Further to this, another example of the usage of CGI::Upload can be found here.

 

perl -le 'print+unpack("N",pack("B32","00000000000000000000001000100000"))'