my @lines;
open my $fh,'<',$file or die $!;
while(<$fh>){
   if(/START/ .. /ENDE/){
       foo();
       push(@lines,$_);
   }
}
close $fh;

print $_ for @lines;

sub foo {
   open my $fh, '<',$otherfile or die $!;
   while (<$fh>) {
      # operations
   }
   close $fh;
}