#! perl -slw
use strict;

my @good = qw[ aa1.2 aa1_bb2 ];
my @bad  = qw[ aa1_2 aa1_2bb 1aa ];

my $re = qr[^[0-9]|^.*[_-][0-9]];

print $_, $_ !~ $re ? ' matches.' : ' doesnt match' for @good, @bad;

__END__
C:\test>junk
aa1.2 matches.
aa1_bb2 matches.
aa1_2 doesnt match
aa1_2bb doesnt match
1aa doesnt match