If all the numbers have the same length, substr is a good tool. Use the four-argument version to modify the string, just add a space at position 8, replacing 0 characters:

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

while (my $number = <DATA>) {
    substr $number, 8, 0, ' ';
    print $number;
}

__DATA__
+12 34567890
+23 45678901
+34 56789012
+45 67890123
+56 78901234
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