Basically, it's Perl shorthand for this:

if ($rows++ and $lsep)
{
  print $fh $lsep;
}
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Meaning, if the two conditions are true, print $fh and $lsep print $lsep to filehandle $fh.

  higle

Update: Doh! Wasn't paying attention at all ($fh should have tipped me that it was a filehandle :c\). Thanks for the correction, RMGir!