my $gnuplot = eval \$buffer->($canvas); 
 #OUTPUT:
 #Undefined subroutine &main::sub {

##</code><code>##

 my $gnuplot = eval \$buffer;  
 $gnuplot->($canvas); 

# OUTPUT
# Use of uninitialized value in subroutine entry at ....
# Undefined subroutine &main:: called at ./zz-std.....