#!/usr/bin/perl -w
use strict;

use constant {    # column indices
    FOO => 0,
    BAR => 1,
    LEN => 2,
};

my @col1;         # '1st-line-of-pair' buffer

while (<DATA>) {  # read line
    chomp;
    my @col2 = split /\t/;      # split line on tabs
    
    if (@col1) {                # two lines read, i.e. pair available?

        if ( $col1[FOO] eq $col2[FOO]
         and $col1[BAR] eq $col2[BAR] ) {   # is pair matching?

            # average length
            $col1[LEN] = sprintf "%.1f", ($col1[LEN] + $col2[LEN]) / 2;

            write_out(@col1);   # write out modified/merged line 
            @col1 = ();         # clear buffer
            next;               # skip rest
        } else {
            write_out(@col1);   # write out non-paired line
        }
    }
    @col1 = @col2;              # store line (previous=current)
}
write_out(@col1) if @col1;      # take care of last line

sub write_out {
    print join("\t", @_), "\n";
}

__DATA__
abc	def	3.5
abc	def	4.5
ghi	jkl	13.2
mno	pqr	2.8
mno	pqr	2.4
stu	vwx	10.0