Take

sub my_function {
    my $self = shift;
    my $parm = shift;
    return $self->{parm};
}
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Now refactor it as

sub my_function { shift->{shift} }
[download]

It looks like it should be the same, but the second shift has now become a literal 'shift' and not a shift function. I wouldn't say it fails spectacularly (well I suppose it could depending on the expected results), but it's darn hard to catch by just looking.