What bothers me a little is this:

while (my @rows = $sth->fetchrow_array() ) {
  return join '', map {sprintf $fmt, @$_ } \@headers, \@rows;
}
$sth->finish;
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That's returning after fetching only a single row, and it also never calls the finish (unless, of course, no rows are returned).

Maybe this would be better?

my $result = sprintf $fmt, @headers;
while ( my @rows = $sth->fetchrow_array() ) {
  $result .= sprintf $fmt, @rows;
}
$sth->finish;
return $result;
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