Perl is mostly case-sensitive. There is some ambiguity over case with regard to module names on case-insensitive file systems. For example, given the following module:

# this code is in Foo/Bar.pm
package Foo::Bar;
use parent qw/Exporter/;
our @EXPORT = qw/baz/;
sub baz { "Hello world"; }
1;
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On a Windows system (Windows a uses case-insensitive but case-preserving file system) you could do this:

use 5.010;
use strict;
use foo::bar;
say Foo::Bar::baz(); # says "Hello world"
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And you could do this:

use 5.010;
use strict;
use Foo::Bar;
say baz(); # says "Hello world"
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But you couldn't do this:

use 5.010;
use strict;
use foo::bar;
say baz(); # dies - undefined subroutine main::baz
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And you couldn't do this:

use 5.010;
use strict;
use foo::bar;
say foo::bar::baz(); # dies - undefined subroutine again
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When you call a subroutine, it must be defined somewhere, unless it is a builtin Perl function, like "each" and "print" in your example. In your code, you want to use functions from the module "Socket.pm" - so you have to "use Socket" to make these functions available.

You can tell that inet_aton is not a Perl function simply by checking with man perlfunc or by trying

perldoc -f inet_aton
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further documentation

• use
• perlsub (perhaps, you should start here)