RMGir explained the problem. Here's a solution:

for(qw(1 4 haha)){
   eval { usenumbers($_) } or die($@);
}

# Returns true on success.
# Throws an exception on error.
sub usenumbers {
   my $var = shift;
   $var=~/^\d+$/
      or die('Invalid argument: Expecting digits');
   print "Yes, [$var] has numbers.";
   return 1;
}
[download]

A variation of the above where the function doesn't need to return true:

for(qw(1 4 haha)){
   eval { usenumbers($_) };
   die($@) if $@;
}

# Throws an exception on error.
sub usenumbers {
   my $var = shift;
   $var=~/^\d+$/
      or die('Invalid argument: Expecting digits');
   print "Yes, [$var] has numbers.";
}
[download]