Then you will have to supply more information because what you've shown so far works as expected:

#! perl -sw
use 5.010;
use strict;

our $FLAG;

if( $FLAG ) {
    require '780835-s.pl';
    doit();
}
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#! perl -sw
use 5.010;
use strict;
use threads;

sub t{
    say 'Hi from thread';
}

sub doit {
    say 'Before';
    my $t = threads->create( \&t );
    say 'After';
}

1;
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C:\test>780835-m -FLAG=0

C:\test>780835-m -FLAG=1
Before
After
Hi from thread
Perl exited with active threads:
        1 running and unjoined
        0 finished and unjoined
        0 running and detached
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