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;
        # ...
    }
}

[download]

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.

In reply to Re: Should we bother to save hash lookups for performance? by Aristotle
in thread Should we bother to save hash lookups for performance? by dlink

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.