If each existing foreach removes more elements than it keeps, the order is not sorted, and memory is not an issue, then

my @tmp;
while (my $elem = pop @array) { # or s/pop/shift/
  # do stuff
  if ($keep) {
    push @tmp, $elem;
  }
}
@array = @tmp;
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Another method is to use an array index instead of foreach, and splice:

my $i=0;
while ($i < @array) {
  if (keep($array[$i]) {
    print "keeping $array[$i]\n";
    $i++;
  } else {
    my $x = splice(@array,$i,1);
    print "discarding $x\n";
  }
}
[download]