You are passing the array to makeSQL as a ref here:

my $sql = makeSql(\@xpid);
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However here you make an error:

sub makeSql {
  my @xpid = shift; # <----------- error
  my $return;       # try: my $xpid = shift
  my $i = 0;                  ^
  foreach my $item (@xpid) { # <-- error
    $return .= $xpid[$i];    # <-- error

# try: foreach my $item (@$xpid)
                         ^
# try: $return .= $$xpid[$i];
                  ^
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The reference the is passed to the function should be stored in a scalar. When it then needs to be used as an array you dereference it with @$xpid and then again with $return .= $$xpid[$i];.