As re provides regular expression matching operations similar to those
found in Perl, I tried there to see what happens:

"""
use Data::Dumper;

$a = 'a\nb\nc';
$a =~ s/$/#/;
print Dumper($a);

$a = 'a\nb\n';
$a =~ s/$/#/;
print Dumper($a);
"""

$ perl pru_sub.pl
$VAR1 = 'a\\nb\\nc#';
$VAR1 = 'a\\nb\\n#';