A few additional tips. The indenting of your code levels should be consistent. The "smallest foreach loop" should be moved to the left to match the level of the "biggest". However, better would be to combine the 2 loops. Also Perl has a special single action syntax putting the "if" at the end of the statement. This can make the code easier to read with fewer curly braces required.

Another big point: What you name variables DOES matter. @unique does not really describe what that is (these are not the uniques of the original array, this is the consecutive range. I would suggest @consecutive as an alternative name.

use strict;
use warnings;

my @arr = (1,2,2,3,4,6);

sub do_it_all 
{
    my $biggest  = $_[0];
    my $smallest = $_[0];
    
    foreach my $num (@_) 
    {
        $biggest  = $num if ($num > $biggest); 
        $smallest = $num if ($num < $smallest);
    }
  
    print "\$smallest = $smallest\t\$biggest = $biggest\n";
    my @consecutive = ($smallest..$biggest);
    print "@consecutive\n";        
}

do_it_all(@arr);

__END__
$smallest = 1    $biggest = 6
1 2 3 4 5 6
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Update: Instead of "do_it_all", perhaps "conseq_range" or similar would be better?