I know this doesn't fit the answer of finding a true once (the examples above me do this well), but maybe you should rethink the way you do your flow. Instead of adding a comma and a value every time the trueonce variable is false, why not push everything onto an array and join it for the final query.

my @values = ();
while($whatever){
  my $value = $got_from_somewhere;
  push @values, $value;
}

my $query = "INSERT INTO somewhere VALUES (";
$query .= join(" ,", @values);
$query .= ")";
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my @a=qw(random brilliant braindead); print $a[rand(@a)];