use strict;
use warnings;
use 5.020; # for best efficiency on $&

use Test::More tests => 4;

my $orig = 'This is a real number, 123456.56';
my $want = 'This is a real number, 493824.56';

(my $have = $orig) =~ s/\d+/sprintf "%i", 4 * $&/e;
my $intpart = $&;
is $intpart, 123456, 'int part captured';
is $have, $want, 'Multiply int part by 4';

($have = $intpart) =~ s/./3/g;
is $have, 333333, 'Int part digits set to "3" trivially';

$want = 'This is a real number, 333333.56';
($have = $orig) =~ s/\d+/(my $int = $&) =~ s#.#3#g; $int/e;
is $have, $want, 'Replace int part in string with all 3s';