my( $Xthread ) = threads->create( { suspended => 1 }, \&Xthread );
my( $Ythread ) = threads->create( { suspended => 1 }, \%Ythread );

... Do other stuff that gets me the parameters for X

$Xthread->resume( $arg1, $arg2 );

... Generate/fetch/calculate args for Y

$Ythread->resume( $Yarg1, $Yarg2 );

... tum te tum

my( @Yresults ) = $Ythread->join;
 ...
my( @Xresults ) = $Xthread->join;