It would probably be better to ask this question at stackoverflow.

Without a file it is kinda difficult to debug, but this may do the trick. You could also just use grep | awk ....

#/usr/bin/perl
use strict;
use warnings;

open(my $FH, <, "file_data.txt") or die "Cannot open file: $!";
LINE: while (my $line = <$FH>) {
    chomp $line;
    next LINE unless $line =~ /^[0-9]/;
    my ($NO, $RES, $DSC_SEC, $PROB_H, $PROB_E, $PROB_C) = split /\s+/,  $line;
    print "$PROB_H\n";
}
close($FH);

Or as suggested keep it simple

[your prompt]$ grep '^[0-9]' yourfile.txt | awk '{print $4}'

to print out the fourth column of lines in yourfile.txt that start with a number

There are a couple of problems with your script. It isn't necessary to read the file into an array as you are going to iterate over the file line by line. Also your test for throwing out non matching lines is only going to match a line starting with "No." and not the other non-matching lines in the file.

     use strict;
     use warnings;

     open(FILE, "file_data.txt") or die "Cannot open file: $!";

     while (my $line = <FILE>) {
#unless the line begins with a number followed by 
#one or more whitespace characters, skip it.
         unless ($line =~ m/^\d+\s+/) {next;}   
         chomp $line;
         my ($NO, $RES, $DSC_SEC, $PROB_H, $PROB_E, $PROB_C) = split(/\s+/, $line);
         print "$PROB_H\n";
        }

    close(FILE);