Here's your problem:

 my ($present_1,$present_2,$present_3,$present_4,$present_5) = $verb;
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I assume you want all of the variables to be initialized with $verb. Howevr, this statement only sets $present_1 and the rest will be set to undef.

A quick fix is to write:

 my ($present_1, ...) = ($verb) x 5;
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Another approach to this would be to first compute the verb stem and then just add the appropriate ending:

  my $stem = $verb;
  $stem =~ s/er$//;

  my $present_1 = $stem . "e";
  my $present_2 = $stem . "es";
  ...
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For your group 1 verbs, this can be very compactly written:

my @present_endings = qw(e es ons ez ent);
my @present = map { $stem . $_ } @present_endings;
...etc for the other tenses...
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In fact, you might be able to make use of the module Lingua::Stem::Fr.