Here is your code:

print STDERR "$rec->[IMP_WG] $rec->[CSC_HAN]\n";
if ( defined( $rec->[IMP_WG] ) &&
     $rec->[IMP_WG] ne '' &&
     defined( $rec->[CSC_HAN] ) &&
     $rec->[CSC_HAN] eq '1' )
{
  print "$rec->[CLASSIF] $rec->[IMP_WG] $rec->[SRV_PLT]\n";
}
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The print statement in the first line will be executed regardlessly. If in deed, $rec->[IMP_WG] and/or $rec->[CSC_HAN] are/is uninitialized, the warning you saw should be issued. To avoid this, simply test the values before you print, as you did in your second line.