It does modify $!, but its only reflected when you Peek if you put $! in int/boolean/string context first, because Peek won't force the conversion. The assignment modified the IV slot, but stat modified the NV/PV slot. 
$ perl -MDevel::Peek -E " stat 1; Dump($!); $!=666; stat 1; $! and Dum
+p($!); "
SV = PVMG(0x9b3bfc) at 0x9ba5ec
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x9d904c
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x9ba61c
    MG_LEN = 1
    MG_PTR = 0x9da014 "!"
SV = PVMG(0x9b3bfc) at 0x9ba5ec
  REFCNT = 2
  FLAGS = (GMG,SMG,pNOK,pPOK)
  IV = 666
  NV = 2
  PV = 0x3ffee4 "No such file or directory"\0
  CUR = 25
  LEN = 28
  MAGIC = 0x9d904c
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x9ba61c
    MG_LEN = 1
    MG_PTR = 0x9da014 "!"

[download]

In reply to Re^5: Different behaviour of 'stat' function after assignment to $! (errno). by Anonymous Monk
in thread Different behaviour of 'stat' function after assignment to $! (errno). by muxxum

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.