A couple of comments first. Standard comment 1 applies: use strict; use warnings;.

More immportant for your future code: you should use the three parameter open open (FH, "<", $path) to avoid the possibility of nasty stuff happening due to bad (in a couple of senses) file names.

Having dealt with that here is some code to start with:

#! /usr/bin/perl
use strict;
use warnings;

print 'Enter path of old file: ';

chomp(my $inPath = <>);
open (inFile, "<", "$inPath") || die "Failed to open in file: $!\n";

print 'Enter path of new file: ';

chomp(my $outPath = <>);
open (outFile, ">", "$outPath") || die "Failed to create out file: $!\
+n";

while (<inFile>){
  $_=~ tr/a-zA-Z/0-310/;
  print outFile " $_\n";
}

close inFile;
close outFile;
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