I've done some debugging to find out why I get this behaiviour.

I've compiled a vanilla perl 5.8.5 with -O0 -g as cflags. It gives the same output as I've shown in the OP.

I've found that $! is directly connected to errno, and so I've put a watchpoint to errno, and found that errno is set to EBADF by the PerlIO_fast_gets function: 

int
PerlIO_fast_gets(PerlIO *f)
{
    if (PerlIOValid(f) && (PerlIOBase(f)->flags & PERLIO_F_FASTGETS)) 
+{
         PerlIO_funcs *tab = PerlIOBase(f)->tab;

         if (tab)
              return (tab->Set_ptrcnt != NULL);
         SETERRNO(EINVAL, LIB_INVARG);
    }
    else
         SETERRNO(EBADF, SS_IVCHAN); // <----

    return 0;
}

[download]
The complete traceback is: 
#0  PerlIO_fast_gets (f=0x817c148) at perlio.c:1721
#1  0x081402e6 in PerlIOBuf_fill (f=0x817ca58) at perlio.c:3576
#2  0x0813da21 in Perl_PerlIO_fill (f=0x817ca58) at perlio.c:1654
#3  0x0813e4ff in PerlIOBase_read (f=0x817ca58, vbuf=0xbfffd657, count
+=1) at perlio.c:2059
#4  0x08140498 in PerlIOBuf_read (f=0x817ca58, vbuf=0xbfffd657, count=
+1) at perlio.c:3624
#5  0x0813d670 in Perl_PerlIO_read (f=0x817ca58, vbuf=0xbfffd657, coun
+t=1) at perlio.c:1564
#6  0x08141f06 in PerlIO_getc (f=0x817ca58) at perlio.c:4771
#7  0x080deb25 in Perl_sv_gets (sv=0x818513c, fp=0x817ca58, append=0) 
+at sv.c:6341
#8  0x080cd19b in Perl_do_readline () at pp_hot.c:1588
#9  0x080c8cbb in Perl_pp_readline () at pp_hot.c:231
#10 0x080b32da in Perl_runops_debug () at dump.c:1442
#11 0x080624c0 in S_run_body (oldscope=1) at perl.c:1924
#12 0x08062053 in perl_run (my_perl=0x8175e70) at perl.c:1843
#13 0x0805e5f4 in main (argc=2, argv=0xbffffa04, env=0xbffffa10) at pe
+rlmain.c:86

[download]

I am not a master of perlio (nor of any other part of perl source), so this still doesn't say much to me, but I hope one of you could help me.

In reply to Re: readline succeeds but sets $! = EBADF by ambrus
in thread readline succeeds but sets $! = EBADF by ambrus

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.