Accumulate the values into a hash where the keys are the 5 minute slots. Split every 3rd set of values between the 2 affected slots. Divide by 5 for the averages:

#! perl -slw
use strict;
use Data::Dumper;

my $re_line = qr[ ( \d\d ): ( \d\d ) ( .* $ ) ]x;

my %timeslots;
my $counter = 0;
while( <DATA> =~ $re_line ) { # Extract hrs, mins, data into $1, $2, $
+3
    my @temp = split ' ', $3;

    for( 0 .. 4 ) {
        # split every 3rd line between 2 5-minute slots
        if( $counter++ % 3 == 2 ) {
            $timeslots{ int(     ( $1 * 60 + $2 ) / 5 ) }->[ $_ ] += $
+temp[ $_ ] / 2;
            $timeslots{ int( 1 + ( $1 * 60 + $2 ) / 5 ) }->[ $_ ] += $
+temp[ $_ ] / 2;
        }
        else {
            $timeslots{ int(     ( $1 * 60 + $2 ) / 5 ) }->[ $_ ] += $
+temp[ $_ ];
        }
    }
}
for my $ts ( sort{ $a <=> $b } keys %timeslots ) {
    printf '%2d:%02d' . ' [%f]' x 5 . $/,
        ( $ts * 5 ) / 60,
        ( $ts * 5 ) % 60,
        map{ $_ / 5 } @{ $timeslots{ $ts } } ;
}

__DATA__
08/07/03.22:55  1.029   1.172   1.03    0.086   0.382
08/07/03.22:57  0.829   2.284   1.219   0.087   0.439
08/07/03.22:59  2.437   0.792   0.809   0.087   0.305
08/07/03.23:01  0.653   1.089   0.541   0.116   0.351
08/07/03.23:03  0.823   2.407   0.826   0.04    0.23
08/07/03.23:05  0.797   1.016   0.619   0.195   0.274
08/07/03.23:07  1.742   0.901   1.078   0.087   0.328
08/07/03.23:09  0.897   1.218   0.512   0.096   0.252
08/07/03.23:11  1.146   1.281   0.521   0.086   0.276
08/07/03.23:13  0.924   1.129   0.891   0.4     0.456
08/07/03.23:15  1.103   1.383   1.645   0.09    0.387
08/07/03.23:17  0.86    2.078   1.098   0.635   0.36
08/07/03.23:19  3.832   1.911   0.808   0.086   0.309
[download]

Output

P:\test>282224
22:55 [0.776100] [0.770400] [0.508600] [0.043300] [0.194700]
23:00 [0.295800] [0.778400] [0.322300] [0.035900] [0.146700]
23:05 [0.679800] [0.525400] [0.388100] [0.070000] [0.143400]
23:10 [0.503700] [0.455500] [0.301100] [0.106800] [0.146200]
23:15 [1.048700] [0.994700] [0.718500] [0.153200] [0.202800]
23:20 [0.110300] [0.207800] [0.080800] [0.009000] [0.036000]
[download]