my %session_start;
my %session_end;
while( <FILE> ) {
    if( /$pattern/ ) {
        # Session starts here
        my $started_session= $1; # actually, you do the counting of parentheses
        my $session_start_time= $2; # actually, you do the counting of parentheses
        if( ! $session_start{ $started_session }) {
            # That session really is new
            $session_start{ $started_session }= $session_start_time;
        } else {
            # That session should be new but we already know it
            warn "Line $.: Session '$started_session' already started at $session_start{ $started_session }, but also (re)starts at $session_start_time. See [$_]";
        };
    } elsif( /(\d+-\d+-\d+ \d+:\d+:\d+).*session:(\S{32})/)
        my $session_end= $1; # actually, you do the counting of parentheses
        my $ended_session= $2; # actually, you do the counting of parentheses
        # Session was seen here, or might end here
        if( ! $session_start{ $ended_session }) {
            # That session is unknown?!really is new
            warn "Line $.: Session '$ended_session' never started, but ends at $session_end. See [$_]";
        } else {
            # That session was seen here
            $session_end{ $ended_session }= $session_end;
        };
    }
};