sub indexBal {
    my( $string, $lookFor, $limit ) = @_;
    my $nesting = 0;
    for( my( $position, $char ) = ( 0, substr $string, 0, 1 );
            $position < min( length( $string ) , $limit );
            $char = substr $string, ++$position, 1
    ) {
            $nesting++ if $char =~ m/[\[\{]/;
            $nesting-- if $char =~ m/[\]\}]/;
            die 'Unbalanced' if $nesting < 0;
            return $position if $char eq $lookFor and $nesting == 0
    }
    return;
}

my $s = '{ [ { [ { x }, { y } ], [ z ] } ] } { [ { ';

my $pos = indexBal $s, '}', length( $s );

print substr $s, 0, $pos+1;

{ [ { [ { x }, { y } ], [ z ] } ] }