You could do something like this ...

$ export ENVONE=HI_THERE
$ env | grep ENV
ENVONE=HI_THERE
$ cat notset.pl 
#!/usr/bin/perl -w
use strict;

my $envOne = $ENV{'ENVONE'};
my $envTwo = $ENV{'ENVTWO'};

print "ENVONE=" . ($envOne||"Is not set") .  
      "ENVTWO=" . ($envTwo||"Is not set") . "\n";

$ ./notset.pl 
ENVONE=HI_THEREENVTWO=Is not set
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