If you don't need this for several variables at once, you can use for as a topicalizer:

sub do_somn {
    my $self = shift;
    for my $username ($self->{username}) {
        my $a = $username;
        my $b = $username;
        # ...
    }
}
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for only aliases the variable to the value. Thus you also avoid the synchonization problems diotalevi mentioned.

In fact I use this relatively often not for performance, but as a form of abstraction. I find

/x/ and $_ .= "y" for $self->{option}; more readable than $self->{option} .= "y" if $self->{option} =~ /x/; And it also adheres to the "do it once and only once" principle. If I change my mind about the hash key's name, I only have one place to update vs 2 (or 3 or 4 or 15..).

Makeshifts last the longest.