my $record_length=200;               # set the record length
open my $fh,'<',$filename or die $!; # open handle
my $size = -s $fh;                   # get the file size
my $records=$size/$record_length;    # workout the number of records
seek $fh, rand($records)*$record_length, 0;# move to a random record
my $randomline = <$fh>;              # read that record
close $fh;                           # Close handle