If this is part of a bigger question and you think some extra effort is worthwhile, then using closures is one way.

use strict;
use warnings;

sub stringmaker {
    my( $start, $stringref ) = @_;
    return sub { $start.$$stringref }
}

my $r = 300;
my $g = stringmaker( 'g', \$r );
print $g->()."\n"; #would be 'g300', that's ok
$r = 500;
print $g->()."\n"; #would be 'g500'
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The disadvantage now is that $g is not a string, but a code reference, and that you need a different stringmaker-routine for different types of strings. The latter can be mitigated somewhat by using sprintf and a template:

use strict;
use warnings;

sub stringmaker {
    my( $template, @vals ) = @_;
    return sub { sprintf $template, map { $$_ } @vals }
}

my $r = 300;
my $i = 0;
my $g = stringmaker( '%s: g%s', \$i, \$r );
print $g->()."\n";  # '0: g300'
$r = 500; $i = 10;
print $g->()."\n";  # '10: g500'
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