# Line 2
my $cleansource = "$course\\$_";
# or: my $cleansource = $course . "\\" . $_;

# Lines 6/7
my $s = "$sourceSize$sourceSizeUnit";
# or: my $s = $sourceSize . $sourceSizeUnit

##</code><code>##

$x = $a; $x += $b; $x += $c; $x += $d;

$x = $a + $b + $c + $d;