First some solutions, then I'll explain. 

map(($_ => 2*$_), @list)
map +($_ => 2*$_), @list
map { $_ => 2*$_ } @list

[download]

In general terms, what's an EXPR and what's not (and why can't $_ => 2*$_ be one)?

You gotta keep in mind the entire list of arguments is also an expression (a list operator), so the expression is bounded by a comma. "=>" is a fancy form of the comma operator.

So why you have is basically a precedence issue. And when you have a precedence issue, you use parens. 

map(($_ => 2*$_), @list)

[download]

map is often used without parens, but that's a problem here since 

map ($_ => 2*$_), @list   # XXX

[download]

means 

map(($_ => 2*$_), ()), @list   # XXX

[download]

The usual trick is to use the unary + operator. 

map +($_ => 2*$_), @list

[download]

Omitting parens around function in Perl is frought with peril.

In reply to Re: map EXPR - what's an EXPR? by ikegami
in thread map EXPR - what's an EXPR? by missingthepoint

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.