Create bigint values explicitly eg.

use Math::BigInt; 
my $big = Math::BigInt->new( "2539885693349640669028422069904549631176
+10523116274484848568321143714323882724980122910689" );
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Computations involving at least one such big integer will get infected and also become bigints. Other values, however, remain plain (non-big) perl number. If you wish to explicitly convert a big value back to a plain number, you can do one of

my $small = $big->numify;
# OR
my $small = "" . $big;
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(this may of course lose precision if that number is large).

Update: you may also restrict use bignum; to smaller scopes, such as bare brace blocks, in which case numeric literals in those scopes are automatically upgraded to big integers.