I'm not sure I understand the "open file handles dynamically" question.. Isn't it always dynamic? If you want to open a list of files, just run in a loop:

foreach my $file (@files)
{
   my $fh;
   open $fh, "<", $file or die "Can't open $file ($!)";
   push @filehandles,  $fh;
}
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If you want to only use 1 filehandle you can always use a mixture of open/seek/tell/close to always re-use a single filehandle, see code below. Of course this is going to be really slow, as you will have to perform all 4 operations on every single line of each file.

Having a pool of open file handles would help some: as long as you are merging less files than the size of the pool you merge then the "natural way" (just read one line at a time from each filehandle), and if you need more, then you use the method below for files that you havent in the pool.

Does it make sense?

#!/usr/bin/perl -w
use strict;

use Fatal qw(open close); # so I don't have to bother testing them

my @files= @ARGV;
my %marker= map { $_ => 0 } @files;

while( keys %marker)
  { foreach my $file (@files)
      { if( exists $marker{$file})
          { open( my $fh, '<', $file);
            seek( $fh, $marker{$file}, 0); # the 0 means 'set the new 
+position in bytes to' $marker{$file}
            if( defined( my $line=<$fh>))
              { print $line;
                $marker{$file}= tell $fh;
              }
            else
              { delete $marker{$file}; }
            close $fh;
          }
      }
  }
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Wow this is so cool! Thanks Boris/C/Mirod!

I intially thought I will just learn how to open multiple filehandles but got lured into implementing the paste command like program (not there yet :)).. Here is my code but for some reason my  $line variable does not get set to undef when 'all' filehandles runs out of data...Should i be checking for something else to terminate the loop?

I guess the code will be much faster if i can generate perl code that puts all the filehandles into one line without looping them through it...

#! /usr/local/bin/perl -w

# Open many file handels

# C's code
foreach my $file (@ARGV) {
   my $fh;
   open $fh, "<", $file or die "Can't open $file ($!)";
   push @filehandles,  $fh;
}

while (1) {
    $line = undef; # Not sure whether this is even required.
    foreach (@filehandles) {
        $line .= <$_>;
        chomp($line);
    }
    print ($line,"\n");
    last if undef($line);
}
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Since the lhs of .= is always a defined value, even if you append undef.


perl -MData::Dumper -e '$x.= undef; print Dumper($x)'
__OUTPUT__
$VAR1 = '';
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Boris

I think you should keep track of which filehandle is still open, and read it only in that case. And I am always wary of while(1) loops. But its probably just me ;--)

#!/usr/bin/perl -w
use strict;

use Fatal qw(open close);

my @files= @ARGV;

my %fh; # file => file handle
foreach my $file (@files)
  { open( my $fh, '<', $file);
    $fh{$file}= $fh;
  }

while( keys %fh)
  { foreach my $file (@files)
      { if( exists $fh{$file})
          { my $fh= $fh{$file};
            if( defined( my $line= <$fh>))
              { chomp $line; print $line;} # regular line
            else
              { delete $fh{$file};       } # eof reached for this file
          }
      }
    print "\n"; # end of line for all files
  }
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