eq.  This:

sub sub do_somn {
    my $self = shift;
    my $username = $self->{username};

    my $a = $username; # for some reason
    my $b = $username; # for some other reason.
    ...
}

 Verse this:

sub do_somn {
    my $self = shift;

    my $a = $self->{username}; # for some reason
    my $b = $self->{username}; # for some other reason.
    ...
}