my $matches = 0;

while (...) {
    ++$matches;
    ...
}

if (! $matches) {
    ... "sorry no matches found"
}
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my $command = $sth->fetchrow_arrayref();
if (! $command) {
    ... "sorry no matches found"
}
else {
    do {
        for (...) {
            printf ...
        }
    } while ($command = $sth->fetchrow_arrayref());
}
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my $first = 1;
while ( (my $command = $sth->fetchrow_arrayref) || $first ) {
    if (! $command) {
        ... "sorry no matches found"
        $first = 0;
    }
    else {
        for ...
    }
}
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$ref is a reference to an array so you can:

if (@$ref) {
   # have elements
} else {
   # No elements to process
}
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See perlreftut and perlref.

When you no longer have any rows fetchrow_arrayref will return undef which will cause problems when you attempt to dereference it. Better to do this implementation: Re: how to determine of fetchrow_Arrayref is empty?.

if ( not( $ref ) ) { 
   print "No results\n";
} else { 
   # code to print results
}
[download]

If I may offer some observations about the way you're doing this:

(1) The variable $numFields would be better named $number_fields. See perlstyle. And you might want to change "$ref" to "$aref" to indicate what kind of reference it is (myself, I'd probably call it $row, though I think Conway prefers $row_ref).

(2) The C-style for loop over the fields might be clearer if done in a more perl like way, just for example:

  my $result = join ", ", @{ $ref }; 
  print $result, "\n";
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(3) Personally I lean towards fetchrow_hashref these days: the performance penalty on that was fixed a long time ago.

fetchrow_hashref wouldn't work with what the OP is doing for each row (at least, not without a lot of hassle).
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The OP hasn't actually made the ultimate goal clear -- my guess would be that a comma separated list of all fields is just an intermediate step while learning how to use DBI.

Starting with fetchrow_hashref and using a stub like this is probably a better place to begin:

foreach my $field_name (keys %{ $row_href }) {
  my $value = $row_href->{ $field_name };
  print "$field_name: $value \n";
}
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And if the order of the fields is important, you can replace keys %{ $row_href } with an array @field_names... typically, you have that information already, since you need it to build the SELECT.

if(!$sth->rows()){
  print STDERR "sorry no matches found\n";
} else {
  # do something useful with data
}
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DBI:

  $rv = $sth->rows;

Returns the number of rows affected by the last row affecting command, or -1 if the number of rows is not known or not available.

Generally, you can only rely on a row count after a non-SELECT execute (for some specific operations like UPDATE and DELETE), or after fetching all the rows of a SELECT statement.

For SELECT statements, it is generally not possible to know how many rows will be returned except by fetching them all. Some drivers will return the number of rows the application has fetched so far, but others may return -1 until all rows have been fetched. So use of the rows method or $DBI::rows with SELECT statements is not recommended.

So it should work to do

if (! $sth->rows) {
   print "sorry, no matching data"
}
[download]

after the while loop.
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