I ended up using $. to keep track of line numbers and a scale widget to select what chunk I want to display. $pointer is the value of the scale. I read the whole file in first and display $pagesize worth of lines. The -to for scale is then computed and the scale callback(getNextPage) configured(not shown here). The response time is really good and the memory usage is kept low also.

while ( <FH> ) {
    $tw->insert('end',$_) if $. <= $pagesize;        
}
        
my $to = ($. % $pagesize == 0) ? int( $. / $pagesize)
                               : int( $. / $pagesize) + 1;
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sub getNextPage {
    my ( $tw ) = @_;
    $tw->delete('1.0','end');
    my $start = ( $pointer -1 )  * $pagesize;
    open FH,$messvar or warn "messvar= $messvar $!\n";
    while ( my $line = <FH> ) {    
        next if $. <= $start;
        $tw->insert('end',$line);
        last if $. == ($pointer * $pagesize);
    }
    $tw->see('1.0');
    close FH;
}
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