while ( ($x) = shift @a ) {
  ...
}

##</code><code>##

  DB<170> @a=1..2
 => (1, 2)

  DB<171> ( ($x) = shift @a ) ? $x : "false"
 => 1

  DB<172> ( ($x) = shift @a ) ? $x : "false"
 => 2

  DB<173> ( ($x) = shift @a ) ? $x : "false"
 => undef                   # true!

##</code><code>##

  DB<174> @a=1..2
 => (1, 2)

  DB<175> ( ($x) = splice @a,0,1 ) ? "$x" : "false"
 => 1

  DB<176> ( ($x) = splice @a,0,1 ) ? "$x" : "false"
 => 2

  DB<177> ( ($x) = splice @a,0,1 ) ? "$x" : "false"
 => "false"

##</code><code>##

The following equivalences hold (assuming "$[ == 0 and $#a >= $i" )

...
                   shift(@a)           splice(@a,0,1)