First I think that this is a typo. It should read:

if ($key =~ /^PERIODEND/ && !$budgetdata) {
    BudgetGroup->budget($input, $myclass, 3)
}
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But this code does work.

my $key = "PERIODEND";
my $budgetdata = 0;

print $key =~ /^PERIODEND/, "\n";
print $budgetdata, "\n";

if ($key =~ /^PERIODEND/ & !$budgetdata) {
    BudgetGroup->budget($input, $myclass, 3)
}
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So this would be true as 1 bitwise and 1 = 1.