Can I have a go? :)

First up, nice and short:

$a .= ".pl";
[download]

then of course there's the ever faithful:

$a = "$a.pl";
[download]

then of course we could get a little tricky:

$a =  pack("A*A*",$a,".pl");
[download]

or just plain ridiculous:

my $x = sub {return shift() . shift()};
$a = &$x($a,".pl");
[download]

and onto the sublime:

$a=~s#$#.pl#;
[download]

rdfield