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.

Update:
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

[download]
The output is still -
#count: 3 tell: 0
#count: 0 tell: 13
#count: 0 tell: 13
#count: 0 tell: 13

[download]
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?

Thanks!

In reply to Re: desire to pass file handles by value by Roger
in thread desire to pass file handles by value by mandog

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