Because no  $ end-of-string anchor is used, the  /^[A-Za-z]{3}\d{3}/ match may allow something improper:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $os_user = 'foo9876543210';
 $os_user =~ /^[A-Za-z]{3}\d{3}/ or die qq{bad user name: '$os_user'};
 print qq{'$os_user' is ok by me!};
"
'foo9876543210' is ok by me!
[download]

See above for a variation that avoids this problem. See also perlre, perlrequick, and perlretut.