$| can never be -1, unless someone has tied it to some class, or has set it via a typeglob: 
tie $|, 'Pkg', ...;

# or

*| = *foo;
$foo = 100;
print $|;  # 100!

[download]
But those are really esoteric cases; and if you're really panicky, just say $| = 0 or $| = 1.

For those interested, $| is as good as a boolean: 

/* in sv.h */
#define IOf_FLUSH  4


/* in mg.c -- the magic file */
/* in Perl_magic_get() */
case '|':
  sv_setiv(sv, (IV)(IoFLAGS(GvIOp(PL_defoutgv)) & IOf_FLUSH) != 0 );
  break;

/* in Perl_magic_set() */
case '|':
  {
    IO *io = GvIOp(PL_defoutgv);
    if ((SvIOK(sv) ? SvIVX(sv) : sv_2iv(sv)) == 0)
      IoFLAGS(io) &= ~IOf_FLUSH;
    else {
      if (!(IoFLAGS(io) & IOf_FLUSH)) {
        PerlIO *ofp = IoOFP(io);
        if (ofp) 
          (void)PerlIO_flush(ofp);
        IoFLAGS(io) |= IOf_FLUSH;
      }
    }
  }
  break;

[download]
Perl_magic_get() ends up returning a 0 or a 1 (which is what you get when you fetch the value of $|). Perl_magic_set() determines whether the value you're setting $| to is true or false, and sets or unsets the IOf_FLUSH bit accordingly.

japhy -- Perl and Regex Hacker

In reply to Re: $ = 0 at start of script by japhy
in thread $|= 0 at start of script by pileswasp

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