use Data::Dump;

my $val = "034";
$val = oct $val if $val=~/^0/;
dd $val;   # 28

# or, for a list of values:
my @vals = qw/ 10 20 0b1101 0xF 023 /;
/^0/ and $_=oct for @vals;
dd @vals;  # (10, 20, 13, 15, 19)