#! 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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