In that case I would load the file into memory as a string (25MB+ alittle bit) and the open that string as a file using perl's "memory file" facility.

I'd then pass the memory filehandle to Tie::File and have it take care of performing the indexing and seeking required to treat the string as an array of lines.

If the file needs to be modified, it just requires that the file be rewound and a single write to update it when processing is finished.

Using T:F's 'memory' option you can decide how much memory you have to trade for speed:

#! perl -slw
use strict;
use Tie::File;

open IN, '<:raw', $ARGV[ 0 ] or die $!;
my $data = do{ local $/ = -s( $ARGV[ 0 ] ); <IN> };
close IN;

open my $fh, '+<', \$data or die $!;
tie my @lines, 'Tie::File', $fh, memory => 20_000_000;

print for @lines[ 100_000, 200_000, 300_000, 400_000 ];

@lines[ 100_000, 200_000, 300_000, 400_000 ] = ( 'modified' ) x 4;

print for @lines[ 100_000, 200_000, 300_000, 400_000 ];

<STDIN>; ## Approx 60 MB here. 25MB file + 20 MB I configured for Tie:
+:File + overhead. 
__END__
P:\test>460532 bigfile.txt
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[download]

this doesn't work for me.. it takes up 56MB when we tie it, but then it seems the sequential access to 100_000 etc, eats memory along the way.. by the <STDIN> line my perl is on 95MB.. read and write seem to eat separately - if I'm evil and ask for $lines[-1] and then write to it, it gets up to 200MB - worse than splitting the slurp...

How long are the lines in your file?

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