my $string = "abe[123955785ada]sdjdjajd";
if ( $string =~ /\[(.*?)\]/ )
{
    my $inside = $1;
    print $inside, "\n";
}

##</code><code>##

    "aslkj[2099asgjskjw]asljgn[awoeiwj]"

##</code><code>##

    2099asgjskjw]asljgn[awoeiwj