I think your edge case is addressed by putting and length( $string ) back into your while conditional.

I prefer a single chain of if/elsifs to nesting, and I factored out the use of $string as it is invariably the first argument to $_print: 

my $string = '';
my $_print = sub {
    print $indent x $_[1], substr( $string, 0, $_[0], '' );
    $string =~ s[^\s*][];
};

sub pp {
    my( $s, $max_width, $EOS ) = @_;
    $string .= $s;
    while ( length( $string ) > ($EOS ? 0 : $max_width) and $string =~
+ m/([\Q[]{},\E])/g ) {

    my ($pos, $first) = ($+[0], $1);

    if ( ! defined $first or $first eq ',' ) {
        $_print->( $pos, $depth );
    } elsif ( $first eq ']' or $first eq '}' ) {
        $_print->( ++$pos, --$depth );
    } elsif ( defined( my $newpos = indexBal( $string, $first eq '[' ?
+ ']' : '}', $max_width ) ) ) {
        $_print->( $newpos + 2, $depth );
    } else {
        $_print->( $pos, $depth++ );
    }
    }
    return;
}

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Caution: Contents may have been coded under pressure.

In reply to Re^2: Refactoring challenge. by Roy Johnson
in thread Refactoring challenge. by BrowserUk

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