From perldata:

"If you evaluate an array in scalar context, it returns the length of the array."

From scalar:

"Forces EXPR to be interpreted in scalar context ..."

Accordingly, scalar @{ $weekly_hash{$i} } will return the number of values:

$ perl -Mstrict -Mwarnings -E '
my %weekly_hash;
my $i = 1;
push @{ $weekly_hash{$i} },"mon","ret","tue","retik","wed","true";
say scalar @{ $weekly_hash{$i} };
'
6
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Your code shows no undefined values. Your text has "... the number of values ...". An undefined value is still a value:

$ perl -Mstrict -Mwarnings -E '
my %weekly_hash;
my $i = 1;
push @{ $weekly_hash{$i} },"mon","ret","tue","retik","wed","true", und
+ef;
say scalar @{ $weekly_hash{$i} };
'
7
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If you really meant that you only wanted a count of defined values, you can do this:

$ perl -Mstrict -Mwarnings -E '
my %weekly_hash;
my $i = 1;
push @{ $weekly_hash{$i} },"mon","ret","tue","retik","wed","true", und
+ef;
say scalar @{[ grep { defined } @{ $weekly_hash{$i} } ]};
'
6
[download]

The count of elements in an array is always available via scalar; there's no need to introduce temporary count variables.