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my $un = shift;
unless( $un ~= s/\A([_A-Za-z][_A-Za-z0-9]+)\Z/$1/ ){
...
$un = lc($un);
...
unless($<){..}

#uid check
unless($< == 0){ # Log attempt and exit; 
...
#carry on
if(defined(getpwnam $un)){ .. }