split /PATTERN/
split   Splits the string EXPR into a list of strings and returns that
        list. By default, empty leading fields are preserved, and empt
+y
        trailing ones are deleted. (If all fields are empty, they are
        considered to be trailing.)
[download]

If there is no leading empty space (field), in his case shouldn't split return just two elements in the array?

split CUTS your string into fields. Between the beginning of string and "WINGFRAME ", there is the empty string. Example

$ perl -e " print qq!($_)\n! for split /,/, q!,1,2,3!"
()
(1)
(2)
(3)

$ perl -e " print qq!($_)\n! for split /,/, q!,,,,1,2,3!"
()
()
()
()
(1)
(2)
(3)

$ perl -e " print qq!($_)\n! for split /(,)/, q!,,,,1,2,3!"
()
(,)
()
(,)
()
(,)
()
(,)
(1)
(,)
(2)
(,)
(3)
[download]

There is an empty string around every corner

$ perl -e " print qq!($_)\n! for split /,/, q!5,,,!"
(5)

$ perl -e " print qq!($_)\n! for split /(,)/, q!5,,,!"
(5)
(,)
()
(,)
()
(,)
[download]

If you don't mind keeping "WINGFRAME " in your elements use zero-width look-behind and -ahead assertions to split at points in the string that are not preceded by beginning of string and are followed by "WINGFRAME ".

$ perl -Mstrict -MData::Dumper -wle '
> my $line = q{WINGFRAME  MACH1;VECTORTHRUST 40 00 40 40;VECTORTHRUST 
+60 00 40 90;VECTORTHRUST 10 40 30 10;VECTORTHRUST 80 10 10 10;VECTORT
+HRUST 60 10 95 00;VECTORTHRUST 70 00 60 00;VECTORTHRUST 80 00 70 90;V
+ECTORTHRUST 60 50 80 90;VECTORTHRUST 60 40 80 10;WINGFRAME  MACH2;VEC
+TORTHRUST 40 00 40 40;VECTORTHRUST 60 00 40 90;VECTORTHRUST 10 40 30 
+10;VECTORTHRUST 80 10 10 10;VECTORTHRUST 60 10 95 00;VECTORTHRUST 70 
+00 60 00;VECTORTHRUST 80 00 70 90;VECTORTHRUST 60 50 80 90;VECTORTHRU
+ST 60 40 80 10;};
> my @arr = split m{(?x) (?<!\A) (?=WINGFRAME )}, $line;
> print Data::Dumper->Dumpxs( [ \ @arr ], [ qw{ *arr } ] );'
@arr = (
         'WINGFRAME  MACH1;VECTORTHRUST 40 00 40 40;VECTORTHRUST 60 00
+ 40 90;VECTORTHRUST 10 40 30 10;VECTORTHRUST 80 10 10 10;VECTORTHRUST
+ 60 10 95 00;VECTORTHRUST 70 00 60 00;VECTORTHRUST 80 00 70 90;VECTOR
+THRUST 60 50 80 90;VECTORTHRUST 60 40 80 10;',
         'WINGFRAME  MACH2;VECTORTHRUST 40 00 40 40;VECTORTHRUST 60 00
+ 40 90;VECTORTHRUST 10 40 30 10;VECTORTHRUST 80 10 10 10;VECTORTHRUST
+ 60 10 95 00;VECTORTHRUST 70 00 60 00;VECTORTHRUST 80 00 70 90;VECTOR
+THRUST 60 50 80 90;VECTORTHRUST 60 40 80 10;'
       );
$
[download]

If you really don't want the "WINGFRAME " text then remove it by substitution in a map.

$ perl -Mstrict -MData::Dumper -wle '
> my $line = q{WINGFRAME  MACH1;VECTORTHRUST 40 00 40 40;VECTORTHRUST 
+60 00 40 90;VECTORTHRUST 10 40 30 10;VECTORTHRUST 80 10 10 10;VECTORT
+HRUST 60 10 95 00;VECTORTHRUST 70 00 60 00;VECTORTHRUST 80 00 70 90;V
+ECTORTHRUST 60 50 80 90;VECTORTHRUST 60 40 80 10;WINGFRAME  MACH2;VEC
+TORTHRUST 40 00 40 40;VECTORTHRUST 60 00 40 90;VECTORTHRUST 10 40 30 
+10;VECTORTHRUST 80 10 10 10;VECTORTHRUST 60 10 95 00;VECTORTHRUST 70 
+00 60 00;VECTORTHRUST 80 00 70 90;VECTORTHRUST 60 50 80 90;VECTORTHRU
+ST 60 40 80 10;};
> my @arr =
>    map { s{WINGFRAME }{}; $_ }
>    split m{(?x) (?<!\A) (?=WINGFRAME )}, $line;
> print Data::Dumper->Dumpxs( [ \ @arr ], [ qw{ *arr } ] );'
@arr = (
         ' MACH1;VECTORTHRUST 40 00 40 40;VECTORTHRUST 60 00 40 90;VEC
+TORTHRUST 10 40 30 10;VECTORTHRUST 80 10 10 10;VECTORTHRUST 60 10 95 
+00;VECTORTHRUST 70 00 60 00;VECTORTHRUST 80 00 70 90;VECTORTHRUST 60 
+50 80 90;VECTORTHRUST 60 40 80 10;',
         ' MACH2;VECTORTHRUST 40 00 40 40;VECTORTHRUST 60 00 40 90;VEC
+TORTHRUST 10 40 30 10;VECTORTHRUST 80 10 10 10;VECTORTHRUST 60 10 95 
+00;VECTORTHRUST 70 00 60 00;VECTORTHRUST 80 00 70 90;VECTORTHRUST 60 
+50 80 90;VECTORTHRUST 60 40 80 10;'
       );
$
[download]

I hope this is helpful.