"perl -e ' $sep=","; while(<>) { s/\Q$sep\E/\t/g; print $_; } warn "Ch
+anged $sep to tab on $. lines\n" ' csvfile.csv > tabfile.tab"
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becomes

#!/usr/bin/perl

use strict;
use warnings;

my $sep=","; 
open (my $csv_file , "<" , 'csvfile.csv');
open (my $tab_file , ">" , 'tabfile.tab');
while(<$csv_file>) { 
   s/\Q$sep\E/\t/g; 
   print $tab_file, $_;
 } 
warn "Changed $sep to tab on $. lines\n"
[download]

However what do you do with lines such as

"Monster, The Cookie", "123 Sesame St, TV land"
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When processing CSV's there are all kinds of edge cases, so use a module that has looked at these issues already rather than ending up with unsupportable code, I would suggest Text::CSV.

You can open a new csv object with a different separator and just write it without having to contemplate cases like the above.