I'm not 100% clear on your question, so I'm assuming you
have a line with:

pref1\t0\n

and you want to be able to change it to:

pref1\t1\n

without rewriting the entire file? If that is correct you can do this which is basically straight out of the Perl Cookbook. Note it uses quite a bit of memory if the file is going to be large.

my $pref = 'pref1'; 

open(FH, "+< $FILE") or die "Error opening $FILE: $!\n"
my @lines = <FH>; 
seek(FH,0,0); 

my $count = 0; 
foreach my $l ( @lines ) { 
    $count++; 
    # Skip lines not matching $pref
    if( $line !~ /^$pref/ ) { next; }

    my ($pref, $num) =~ /^(.*?)\t(.*?)$/; 
    $num = 1; 

    $lines[$count] = "$pref\t$num\n"; 
}

print FH @lines; 
truncate(FH, tell(FH)); 
close(FH);
[download]

-----------------
Frank Wiles <frank@wiles.org>
http://frank.wiles.org

Edit kudra, 2001-07-30 Changed pre tags to code tags