Dear Monks,
I do get the data I need form the following code.
However the warnings probably indicates that there is something wrong with my structure.

use warnings;
use strict;

my $line;
my $msgtype;
my $bytes34 ;

while (<DATA>) {
    chomp;
    next if ($_ =~ /^$/); # fixes the issue...
    $line = $_;
    $msgtype = substr $line, 6, 8;
    
    # keep only relevant type
    next if ( $msgtype ne "18FEF200") ;
    
    $bytes34 = substr $line, 26, 2;
    print "Line is: $line\n";
    print "qty is: $bytes34\n";
}

__END__
 0    18FEF200 X       8  00  00  FF  FF  00  00  FF  FF   17487.23877
+0 
 0    18FEF200 X       8  00  00  FF  FF  00  00  FF  FF   17487.13877
+0 
 0    18FEF200 X       8  0F  00  00  00  00  00  FF  FF   17484.93861
+0 
 0    18FEF200 X       8  12  00  00  00  00  00  FF  FF   17484.83858
+0 
 0    18FEF200 X       8  00  00  FF  FF  00  00  FF  FF   17486.83875
+0 
 0    18FEF600 X       8  FF  31  72  FF  FF  FF  FF  FF   17486.75931
+0 
 0    18FEF200 X       8  1E  01  00  00  00  00  FF  FF   17321.32261
+0 
 0    18FEF200 X       8  2E  01  00  00  00  00  FF  FF   17321.22202
+0 
 0    18FEF200 X       8  45  01  00  00  00  00  FF  FF   17321.12260
+0 
 0    18FEF200 X       8  5A  02  00  00  00  00  FF  FF   17016.19730
+0
 0    18FEF200 X       8  75  02  00  00  00  00  FF  FF   17016.09727
+0
[download]

Yields the 10 correct values followed by 5 warnings such as:

substr outside of string at parsefile.pl line 13, <DATA> line 12.
Use of uninitialized value $msgtype in string ne at parsefile.pl line 
+16, <DATA>
 line 12.
[download]

Your indulgence and any hints will be greatly appreciated!
**update**
next if ($_ =~ /^$/); inserted to address possible empty lines, thanks to all