Technically in perl-speak, that is the typeglob way to create a read-only variable, not a constant

This is a constant  sub var() { 42 } print var;

This is a read-only

$ perl -Mstrict -le " our $var; *var= \3.14159; print $var; $var++ "
3.14159
Modification of a read-only value attempted at -e line 1.
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This is fast read-only

$ perl -le " use Attribute::Constant; my $pi : Constant( 3.14159265358
+9793 ); print $pi; $pi++"
3.14159265358979
Modification of a read-only value attempted at -e line 1.
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OTOH

$ perl -Mbignum=PI -le " print PI() "
3.141592653589793238462643383279502884197
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