open( OUT, ">some_file.txt" ) or die $!;
my $startline = 1;
my $linespan = 100;

while (1) {
    my $startid = sprintf( "%.1f", $startline );
    my $txt = $text_widget->get( $startid, "+ $linespan lines" );
    last unless length( $txt );
    print OUT $txt;
    $startline += $linespan;
}
close OUT;