while (split(/[, ]+/, <DATA>))
{
  ...
}
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Only a while-loop condition expression like

while (<DATA>)
{
  ...
}
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assigns implicitly to $_ (see tybalt89's example) (update: but see kcott's post for a more complete discussion of $_ assignment special-casing). An arbitrary expression like split(...) does not. Had you had warnings enabled, Perl would have at least hinted at this problem.