#  Assuming 'blargh' is an unreachable host.
perl -MNet::SMTP -e '$m = Net::SMTP->new( "blargh" ); $m->mail( $EVN{USER} )'

#  produces "Can't call method "mail" on an undefined value at ...".

##</code><code>##

Net::SMTP->new( 'blargh' )
  or die $!;

#  produces "Invalid argument at -e line 1".

##</code><code>##

Net::SMTP->new( 'blargh' )
  #or die
  or die $@
  ;

#  produces "Net::SMTP: Bad hostname 'blargh' at -e line 1.".