Roy Johnson,
I understand it is un-tested, so some nits:

• When you call the output_summary sub, you need to use () as a suffix or & as a prefix unless it has been previously seen.
• You forgot to chomp newlines
• ++$rowcount not +$rowcount
• ++$ec{$e} not ++%ec{$e}
• $min and $max need to have initial values prior to doing numerical comparisons

Finally, I don't think it is working correctly - shouldn't the max for :20 be 4.151?.
Here is my stab at it:

#!/usr/bin/perl
use strict;
use warnings;

my %data;
while ( <DATA> ) {
    chomp;
    my ($stamp, $reading) = split;
    next if ! $stamp || ! defined $reading;
    $data{$stamp}{COUNT}++;
    $data{$stamp}{MAX} = $reading if ! defined $data{$stamp}{MAX} || $
+reading > $data{$stamp}{MAX};
    $data{$stamp}{MIN} = $reading if ! defined $data{$stamp}{MIN} || $
+reading < $data{$stamp}{MIN};
    $data{$stamp}{SUM} += $reading;
}
print join "\t" , "TIMESTAMP", "MAX", "MIN", "AVG";
print "\n";
for my $stamp ( sort keys %data ) {
    print join "\t", $stamp, @{$data{$stamp}}{ qw(MAX MIN) }, $data{$s
+tamp}{SUM} / $data{$stamp}{COUNT};
    print "\n";
}
[download]

L~R