$requirement=~s/. /.\\n/g; $requirement=~s/, /,\\n/g;
...
... if it's taken from an array the balloon is shown as a single line where \n is shown as a part of string.

Something like  $requirement=~s/. /.\\n/g substitutes a literal  '\n' (backslash character '\' followed by an 'n' character) into the  $requirement string. You want  \n instead:
    $requirement=~s/. /.\n/g;
(Update: Remember that the replacement field of the  s/// operator follows double-quote string interpolation rules unless  '...' (single-quote) delimiters are used.)

Update: A more concise and general substitution would be
    $requirement =~ s{ (?<= [,.]) [ \t] }{\n}xmsg;
to cover both cases in one swell foop.