Definitely one of those things that appears on the surface to be trivial, but turns out to be much harder:

Here's one way

#! perl -slw
use strict;
use Data::Dump qw[ pp ];

my %hash;
while( <DATA> ) {
    chomp;
    my( $pre, $temp ) = m[(^.+?)(\d+|\D)$];
    $temp = $pre . ++$temp;

    if( exists $hash{ $_ } ) {
        push @{ $hash{ $_ } }, $_;
    }
    elsif( exists $hash{ $temp } ) {
        push @{ $hash { $temp } }, $_;
    }
    else {
        $hash{ $temp } = [ $_ ];
        $hash{ $_ } = [ $_ ];
    }
}

print join ';', @{ $_ }
    for grep{ @{ $_ } == 2 } values %hash;

__DATA__
AAA30
BBC5
SHT12H
DAL33B
BBC49
AAA31
DAL33A
BBC6
SHT12G
BBC50
[download]

Output:

C:\test>junk
DAL33B;DAL33A
AAA30;AAA31
BBC49;BBC50
BBC5;BBC6
SHT12H;SHT12G
[download]

If you need the doubled-up output, just print them twice with the elements reversed.