Re-reading your problem (and if I understand it correctly), I'd do it this way:

my %file;
@routers{@ARRAY} = undef;
die "Bad zero_arg" unless exists $routers{$zero_arg};
my %interfaces;
@interfaces{@anotherarray} = undef;


unless (exists $interfaces{$sec_arg)) {
 #Do stuff
}
# OR this, if appropriate
return if exists $interfaces{$sec_arg};
#Do stuff
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