for($i = 0; $i < @lines; $i++) {
  print $lines[$i] if $lines[$i] =~ /whatever/;
}
# or 
for(0..$#lines) {
  print $lines[$_] if $lines[$_] =~ /whatever/;
}
# or 
my $i = 0;
for(@lines) {
  print $i if /whatever/;
  $i++;
}