If the my @l_UserDetails is declared at main scope, the array inside the hash is indeed the same array.

To give it a life of its own instead, a copy of the array first has to be declared with dynamic scope:

my %g_User;
my @l_UserDetails = ('hallo','goodbye');
InsertArray( \@l_UserDetails, \%g_User, 'user1' );
sub InsertArray {
    my $aref = shift;
    my $href = shift;
    my $elt = shift;
    my @dynamic = @$aref;
    $href -> { $elt } = \@dynamic;
}
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