use strict;
use warnings;
use 5.010;
 
my $foo = "1,2,3";
substr($foo, rindex($foo,","), 1, "|");
say $foo;   # 1,2|3
 
$foo = "1|2|3";
substr($foo, rindex($foo,"|"), 1, ",");
say $foo;   # 1|2,3
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