open FILE, '<', $file or die "...";
while (<FILE>) {
  print if m/interesting/;
  handle(FILE, 'that') if m/that/;
}
close FILE;

##</code><code>##

open my {I/O}FILE, '<', $file or die "...";
while (<{I/O}FILE>) {
  print if m/interesting/;
  handle({I/O}FILE, 'that') if m/that/;
}
close {I/O}FILE;

##</code><code>##

open my {I/O}FILE, '<', $file or die "...";
while (defined($_ = {I/O}FILE->readline)) {
  print $_ if $_ =~ m/interesting/;
  handle({I/O}FILE, 'that') if $_ =~ m/that/;
}
{I/O}FILE->close;

##</code><code>##

sub handle {
  my $fh = shift;
  my $what = shift;

  ...
}