use warnings;
use strict;

open my $rndLines, '<', "rand_sorted.txt" or die "Can't open rand_sorted.txt: $!";

while (defined (my $nextLine = <$rndLines>)) {
    chomp $nextLine;
    
    next unless $nextLine =~ /^\d+/;
    
    my $line;
    while (defined ($line = <>)) {
        last if $. >= $nextLine;
    }
    
    print $line if defined $line;
}

close $rndLines;