$p_str = join ",",'?'x@array;

Sandy's explanation is almost correct. The problem is that in the OP, there are parentheses around the '?' string. Those parentheses matter, because they cause the x operator to act on a list instead of a scalar. So actually, it's more like

my @array = qw(a b c);
my @tmp = ('?') x @array; # @tmp now contains ('?', '?', '?')
my $p_str = join ",", @tmp; # $p_str now contains '?,?,?'
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