Your $query is failing and you don't have RaiseError set on the DBI handle. Store the result to another variable and check if it's undef or not; if it is, check $dbh->errstr to see what went wrong.

my $res = $dbh->selectcol_arrayref( $query );
unless( defined $res ) {
  die "Error executing '$query': ", $dbh->errstr, "\n";
}
my $welcome = pop @{ $res };
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Also of use may be using the trace method on your handle to watch exactly what's being sent back and forth to your underlying database.

Update: Heh. So . . . do you perhaps think you should check if you're getting undef back?