tr doesn't do interpolation: 
my $line = "ABABABAB";
my $limit = 3;
my $base = "A";

say "Yes 1" if $line =~ tr/$base/$base/ >= $limit;
say "Yes 2" if eval "\$line =~ tr/$base/$base/" >= $limit;
__END__
Yes 2

[download]
You need an eval to interpolate.

In reply to Re^2: searching characters by JavaFan
in thread searching characters by Anonymous Monk

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