use strict;
use warnings;

while (defined (my $line = <DATA>)) {
    next unless $line =~ /^(catch\s*)\[(.*)/s;

    my $prefix = $1;
    my ($body, $tail) = parse ($2);

    $line = "${prefix}{$body}$tail";
} continue {
    print $line;
}


sub parse {
    my ($tail) = @_;
    my $outStr = '';

    while (defined $tail) {
        if ($tail =~ s/^([^[]*)\[(.*)//s) {
            my $prefix = $1;
            my $body;

            ($body, $tail) = parse ($2);
            $outStr .= "$prefix [$body]";
            next if length $tail;
        } elsif ($tail =~ s/^([^\]]*)](.*)//s) {
            return "$outStr$1", $2;
        }

        $outStr .= $tail;
        $tail = <DATA>;
    }

    return $outStr, '';
}


__DATA__
catch [ this; that; the; other; ]

some random stuff that isn't a catch

catch [
this[];
that + 1;
the other;
]

##</code><code>##

catch { this; that; the; other; }

some random stuff that isn't a catch

catch {
this [];
that + 1;
the other;
}