You may find:

use strict;
use warnings;

1 while defined ($_ = <DATA>) and ! m/^REGULATION SLOT/;

while (<DATA>) {
        my @Elements = split ";";
        my $element = $Elements[1];
        my @Parts = split " ", $element;
         
        print join (';', @Parts), "\n";
    }

__DATA__
Header line to be skipped

REGULATION SLOT LAYOUT + REGULATED FLIGHTS
element 1; element 2
[download]

a somewhat cleaner technique. Note that the I/O using external files has been replaced by DATA and STDOUT for purposes of demonstration.