use warnings;
use strict;

my $foo = "1,2,3";
print modify( $foo, ',', '|' );  ## prints 1,2|3

$foo = "1|2|3";
print modify( $foo, '|', ',' );  ## prints 1|2,3

sub modify {
    my ( $val, $sep, $rep ) = @_;
    $val = scalar reverse $val;
    $val =~ s/\Q$sep\E/$rep/;
    return scalar reverse $val;
}