in reply to desire to pass file handles by value

I think you *need* to rewind the file by calling seek FH, 0, 0; in the subroutine find.

The file operation in Perl is ultimately linked to the underlying unix file handles. When you read from a Perl file handle, Perl uses and updates the underlying unix file handle, thus giving the side effect.

I think ideally you are looking for the Perl's equivalent to the C stdio's dup function, to duplicate/copy an existing file handle into a new file handle (not just an alias to the existing file handle, but a new independent file handle), and operate on the second file handle in the subroutine. I am not aware of such facility in Perl 5 (my lack of research perhaps), but I think Perl 6 has implimented the dup function for the File Object.

Ok, I just learned that you can duplicate a file handle in Perl with open NEW, "<&OLD";. I quickly came up with the following code -

#!/usr/bin/perl -w use strict; sub find { local *IN = shift; my $find_me = shift; my $count = 0; # duplicate existing file handle open F2, '<&', *IN or die "Can not duplicate file handle"; my $tell = tell(F2); while (<F2>) { $count++ if (/$find_me/); } return "count: $count tell: $tell\n"; close F2; } open OUT, '>', 'tmp.txt' or die "$!\n"; print OUT while (<DATA>); close OUT or die "$!\n"; open IN, 'tmp.txt' or die "$!\n"; print find( *IN, 'a' ); print find( *IN, 'a' ); print find( *IN, 'd' ); print find( *IN, 'e' ); __DATA__ a a a c d e
The output is still -
#count: 3 tell: 0 #count: 0 tell: 13 #count: 0 tell: 13 #count: 0 tell: 13
That didn't work either! Ok, that taught me a lession - my assumption on the duplicated file handle could be wrong. I need some re-education.

Fellow monks, could you please tell me what is wrong with the duplicated file handle? Is perl actually creating a second independent file handle? Am I doing the right thing at all?