sub function_1 { print "One\n" }
sub function_2 { print "Two\n" }

{     # Allow symbol table manipulation.
      # You do use strict;, right? :)
    no strict 'refs';

    foreach my $no ( 1 .. 2 ) {
        &{ "function_$no" }; # <--- THIS THING
    }
}

##</code><code>##

sub function_1 { print "One\n" }
sub function_2 { print "Two\n" }

my %functions = (
    'DEFAULT' => sub { print "Huh?\n" },
    1         => \&function_1,
    2         => \&function_2,
);

foreach my $no ( 1 .. 3 ) {
    $no = 'DEFAULT' unless exists $functions{$no};

    $functions{$no}->();
}