Here is one approach.

    #!/usr/bin/perl -w

    use strict;

    my @leftCon  = qw(left123  left456);
    my @rightCon = qw(right123 right456 right789);

    my %l_con;
    my %r_con;

    @l_con{@leftCon}  = (1 .. @leftCon);
    @r_con{@rightCon} = (1 .. @rightCon);


    while (<DATA>) {
        next unless /:/;
        chomp;

        my ($ping1, $ping2) = split /:/;

        $ping1 =~ s/(?<=\d\d\d)\d+$//;
        $ping2 =~ s/(?<=\d\d\d)\d+$//;

        s/$ping1/$l_con{$ping1}."L"/e if defined $l_con{$ping1};
        s/$ping2/$l_con{$ping2}."L"/e if defined $l_con{$ping2};
        s/$ping1/$r_con{$ping1}."R"/e if defined $r_con{$ping1};
        s/$ping2/$r_con{$ping2}."R"/e if defined $r_con{$ping2};

        print $_," ";
    }

    print "\n";


    __DATA__


    left1232:right4562
    right1234:right4563
    right7895:left4565
    right7891:left1232
[download]

Prints:

    1L2:2R2 1R4:2R3 3R5:2L5 3R1:1L2
[download]

One aspect of the problem that wasn't well defined was whether left12345 can be interpreted as both left123 45 and left1234 5. I took the statement above that the connection names would contain 3 digits as the defining case.

Also, there is some duplicated code here that could probably be simplified upon further clarification.

--
John.