It is a one-liner, because I wouldn't bother filing it as another script.

####################################

C:\>ftype /?

    ASSOC .pl=PerlScript
    FTYPE PerlScript=perl.exe %1 %*

would allow you to invoke a Perl script as follows:

    script.pl 1 2 3

If you want to eliminate the need to type the extensions, then do the 
+following:

    set PATHEXT=.pl;%PATHEXT%

and the script could be invoked as follows:

    script 1 2 3
[download]

Here's your code reformatted:

#!/your/perl/here

use strict;
use warnings;

my @precio;

open PR,'medicines.txt';
@precio=<PR>;
close PR;
chomp(@precio);

print("\n** MEDICINE PRICES BY DOSES PER MONTH **\n\n");

my $acumes;

foreach my $li (@precio)
{
    next if ($li=~/^[_#\W]/);
    my ($med,$uni,$pre)=split(/\t/,$li);
    my $dos;
    print('Daily dose of ',$med,'? =>');
    $dos=<>;
    $dos=~s/[^\d\.]//g;
    my $tot=sprintf("%6.2f",$pre/$uni*$dos*31);
    print('  Costs.................'," \t\t\t$tot \$\/month.\n");
    $acumes+=$tot;
}

my $lit="\nTotal to spend with medicines every month:      $acumes \$\
+n";
print('=' x length($lit),$lit, '=' x length($lit),"\n");
[download]

About passing the input file on the command line, it would be an interesting issue since I would then need to use the <STDIN> diamond to ask for the quantities. If not, it would mess with the previous <> input ...

#!/your/perl/here

use strict;
use warnings;

while (<>)
{ 
    print "one: $_";
}

print "\n\n: now reading from STDIN: \n\n";

print "Enter data: ";
while (<>)
{ 
    print "two: $_";
    print "Enter data: ";
}
[download]

I'm suggesting that some medications come in different pill concentrations, and you might save money by shopping around. And you might program that by using a file for the prescribed doses, and another file for the concentration/price tables.

Now, I've left something out. You could do this by passing 2 files on the command line, and making judicious use of the continue and eof elements. Just check out eof for more details.

Cheers,