You can use a regular expression to look for the pattern, and an until loop to keep prompting the user until their input matches your desired pattern: 
my $os_user = '&'; #stop warnings from yelling at you
print "OS username:";
until ($os_user =~ /^[A-Za-z]{3}\d{3}$/) {
  chomp($os_user = <>);
}

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In reply to Re: Checking name convention by Stringer
in thread Checking name convention by zalezny

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