That's a very entertaining question!  The reason it took me a while to answer is was that I couldn't remember the overriding mistake that I make, out of the many that I make all the time.  I knew there was something, but I just couldn't put my finger on it.

Then I remembered ... the hands-down winner for me is putting semi-colons after values in a declared hash, instead of commas.

That is, I like to write long data structures in this format:

my $p_employee = {
    'alice' => {
        'salary'  => 75000,
        'hired'   => 1176649545,
        'manager' => 'bob',
    },
    'bob' => {
        'salary'  => 82000,
        'hired'   => 1145113545,
        'manager' => 'carol',
    },
    'carol' => {
        'salary'  => 96000,
        'hired'   => 1145113545,
        'manager' => 'thomas',
    },
};
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But invariably I end up putting semi-colons at the end of one or more lines -- for example:

my $p_employee = {
    'alice' => {
        'salary'  => 75000,
        'hired'   => 1176649545,
        'manager' => 'bob',
    };
    'bob' => {
        'salary'  => 82000,
        'hired'   => 1145113545,
        'manager' => 'carol',
    };
    'carol' => {
        'salary'  => 96000,
        'hired'   => 1145113545,
        'manager' => 'thomas',
    },
};
[download]

It also seems to happen with a frequency in direct proportion to the complexity of the data structure.  I'm pretty sure this has bitten me more than any other single class of error.