Well that looks like a line you seen in alot of subroutines eg 
sub do_something {
 my $classname = shift;
....
}

do_somthing($something);

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All it does is take the first element off the array @_ (which in this case contains the elements of the array passed to the subroutine) and puts that value into $classname

From the perldoc

See also `unshift', `push', and `pop'.  `Shift()'
and `unshift' do the same thing to the left end of
an array that `pop' and `push' do to the right end.

Diarmuid

In reply to Re: What does the SHIFT bit do in a constructor by diarmuid
in thread What does the SHIFT bit do in a constructor by Fian

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