As detailed in DESCRIPTION in perlsub, variables passed into a subroutine are stored in the special @_ array. The shift at the start of your subroutine implicitly accesses this array. Since $nic is presumably a hash reference and thus a scalar, you could access it with:

sub getPlatformFiles {
    my $plat_in = shift;
    my $nic = shift;
    my @list;
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A style I prefer when not using OO-style programming is to use an list assignment rather than a shift, but this is wholly subjective:

sub getPlatformFiles {
    my ($plat_in, $nic) = @_;
    my @list;
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There are some potential complexities involved in scoping here, which may bite you if you are incautious and use the same variable names in a script and a subroutine. Note the following is for demonstration purposes and using this approach without good reason can result in some painful bug hunting. If you declare a variable with my in scoping using that contains a subroutine, that subroutine can see all variables in that scope, and hence the following works:

#!/usr/bin/perl
use strict;
use warnings;

my $variable = "Hello\n";
outputter();

sub outputter {
    print $variable;
}
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This is because the scope of $variable is at the script level and contains the subroutine definition. On the other hand, the following will output an error message:

#!/usr/bin/perl
use strict;
use warnings;

sub outputter {
    print $variable;
}

my $variable = "Hello\n";
outputter();
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because the variable is not declared prior to its use in the subroutine. These behaviors allow for some very powerful structures called closures.