I would like it to look something like this.
my $str = 'axxxx';
my $replacement = 'string_begins_with_a';
$str =~ s/^a/$replacement/;
print "$str\n"; #should print 'string_begins_with_a'
Match the whole string in regex using
^a.* and then replace it using your replacement string.
$str =~ s/^a.*/$replacement/; print "$str\n"; # would print 'string_begins_with_a'