use strict;
use warnings;

open my $fh, .... or die "Err: $!";

while (my $line = <$fh>) {
  if ( $line =~ /...some pattern.../ ) {
    print $line;
  }
}
close $fh;

##</code><code>##

use strict;
use warnings;

open my $fh, .... or die "Err: $!";

while (my $line = <$fh>) {
  print $line;
}
close $fh;