Does the x operator help?

my @placeholders = ('?') x $n;
my $str = sprintf <<'SQL', join ",", @placeholders;

    CALL DB_PROC_FOO(%s);

SQL
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Sometimes I also use <<'' instead of <<'SQL' but I find that I tend to insert blank lines into my SQL to structure it better, so that's a practice that leads to inconvenient errors.

Maybe you want to give SQL::Abstract a try, but in the long run, I found that I ended up fighting SQL::Abstract's ideas of how I should structure my data to get the SQL I want, and it was faster for me to just interpolate a string.