Generally I can find and fix a problem like this. But 
this time it seems like something else is involved. The message, in 
the title of this post, appears from only line 123, and 
not on the previous lines. I do this same type of 'if' 
in other places with no messages. 

When I copy the code to a small test case, it works fine. I have 
verified with the debugger and print statements that the
variables involved are defined.

for ($sdd=0; $sdd <= $#gSDDdata; $sdd++) { # foreach SDD
  
  for ($srs=0; $srs <= $#gSRSData; $srs++) { # foreach SRS

    if (not defined $gSDDdata[$sdd]{source}) {
      print "m \n";
    }
    elsif (not defined ($gSRSData[$srs]{sect})) {
      print "n \n";
    }

    if ($gSDDdata[$sdd]{source} =~ /$gSRSData[$srs]{sect}/) { # <- 123

      $gSRSData[$srs]{found} = $gSDDdata[$sdd]{found} = 1;

    }
  }
}