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   17487.238770 
 0    18FEF200 X       8  00  00  FF  FF  00  00  FF  FF   17487.138770 
 0    18FEF200 X       8  0F  00  00  00  00  00  FF  FF   17484.938610 
 0    18FEF200 X       8  12  00  00  00  00  00  FF  FF   17484.838580 
 0    18FEF200 X       8  00  00  FF  FF  00  00  FF  FF   17486.838750 
 0    18FEF600 X       8  FF  31  72  FF  FF  FF  FF  FF   17486.759310 
 0    18FEF200 X       8  1E  01  00  00  00  00  FF  FF   17321.322610 
 0    18FEF200 X       8  2E  01  00  00  00  00  FF  FF   17321.222020 
 0    18FEF200 X       8  45  01  00  00  00  00  FF  FF   17321.122600 
 0    18FEF200 X       8  5A  02  00  00  00  00  FF  FF   17016.197300
 0    18FEF200 X       8  75  02  00  00  00  00  FF  FF   17016.097270