The error is pretty self-describing. $var contains undef. If you don't want undef to be passed to the invocation of $funct, you can find out where it is passed by using:

use Carp 'croak';

my $funct= sub {
    my $var= shift;
    croak '$var was passed an uninitialized value'
        if not defined $var;
    ...
};
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Note that the prototype on the subroutine won't help anything because a prototype only influences how Perl parses source code and Perl does not know where the value of $funct is used to call a function. I would leave the prototype off.

Hello CropCircle, and welcome to the Monastery!

This line:

my $var2 = $var =~ /red|green|blue/;
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looks a bit strange to me. If $var contains 'xredy', for example, the match will succeed but $var2 will be set to 1 (“true”):

22:35 >perl -wE "my $var = 'xredy'; my $var2 = $var =~ /red|green|blue
+/; say qq[|$var2|];"
|1|

22:35 >
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Is that really what you want? To set $var2 to the string that was matched, you need something like this:

22:25 >perl -wE "my $var = 'xredy'; my ($var2) = $var =~ /(red|green|b
+lue)/; say qq[|$var2|];"
|red|

22:30 >
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The parentheses within the regex are for capturing, and the parentheses around $var put the match into list context. See perlretut#Extracting-matches. Or you can get the capture in $1:

22:38 >perl -wE "my $var = 'xredy'; $var =~ /(red|green|blue)/; my $va
+r2 = $1; say qq[|$var2|];"
|red|

22:38 >
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Hope that helps,

Thanks a lot!

The code you have posted does not produce that warning.

$ perl -w
  my $funct  = sub($) {
        my $var = shift;
        my $var2 = $var =~ /red|green|blue/;
        $var2 = 'magenta' if ($var eq 'm');
        return $var2;
    };
$
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Just compiling the sub ref will not produce that warning anyway. The sub ref needs to be actually called by some other code and executed to get an unitialized value warning.
Je suis Charlie.