$NEXTDAY is NOT the same variable as $nextday. The following example uses strings as the values in the variables, but using numeric values (with appropriate adjustment of the test at Ln 8) would provide a similar demonstration.

#!/usr/bin/perl
use 5.014;
# 987079

my $NEXTDAY = "fooday";
my $nextday = "BARDAY";

if ($NEXTDAY eq $nextday) {
    say "Behold, a miracle!";
} else {
    say "\$NEXTDAY is $NEXTDAY and \$nextday is $nextday";
}
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If you used the == test at Ln 8, you would observe an apparent miracle... but only because an == test checks for numeric equality... even when applied to non-numerics (eg, strings).