perhaps this is better.

• the subs are anonymous and scoped within the pattern
• $1 is used instead of $+. why are you using $+ anyway?
• the pattern is commented, increasing clarity

#!/usr/bin/perl
use strict;
use warnings;

$a = "12.2424.4812367";

$a =~ s/(?x)
    (
        (?# match integer and floating point numbers )
        (?: \d+ (?: \. \d+ )? )
    )
    (
        (?# execute code, match result )
        (??{
            ## return twice the first argument
            sub{ shift() * 2 }->( $1 ) 
        })
    )
    /
        ## print the arguments as a couple
        sub{ join( '', '(', shift(), ',', shift(), ')' ) }->( $1, $2 )
    /ge;

print $a;
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~Particle *accelerates*