Our own functions are easy to recognice due to the fact we have to program conform to a predefined coding standard.
eg:
funcions are named using camelnotation, we use an indentation of 4 spaces(tab) 'private' functions have an _ prefix (i know they do not really exist in perl, but this way we know a function is ment to be private) etc.
Next we allways use strict and have warnings on, and we code with the idea that readability is more important than a fast and unreadable algorithm, and we try to avoid perlmagic as much as possible.

package Just_A_Package_Name
use strict;

sub _privateFunctionName() {
    my $self = shift;
    my ($otherItem) = @_;
}

sub publicFunctionName(){
...
}

# So we'd rather write
foreach my $item (@array) {
# than
foreach (@array) {
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