I've edited my post because putting the (?{ }) block after trying to match 4.1.2 was the obvious reason why it didn't called again. But I've tried this: 

use v5.20.0;

my @stoppedStandardServices = ( 'OP OPC Client', 'OP Calculation Serve
+r', 'OP Data Server', 'OP Configuration Server', 'OP Log Server', 'OP
+ Time Server', 'ONC RPC PortMapper');

my $supported = '4.0.0,4.0.1,4.1.0,4.1.1,4.1.2';

$supported =~ /.(?{say '>>> In the regex'})/ if @ARGV;
"4.1.2" =~ //;

say "Start Basic Standard Services:";
startStandardServices( 'Config');
say "Start Remaining Standard Services:";
startStandardServices();

say "Installation completed";

sub startStandardServices {
    my $last = shift;
    my $name;
    while (@stoppedStandardServices) {
        $name = pop @stoppedStandardServices;
        say '  Starting $name=\'', $name, '\' $last=', defined $last ?
+ "'$last'" : 'undef', ' ($name=~/$last/i)=', ($name =~ /$last/i) ? 1 
+: 0;
        last if $name =~ /$last/i;
    }
}

[download]
And with one parameter I get the output: 
>>> In the regex
>>> In the regex
Start Basic Standard Services:
  Starting $name='ONC RPC PortMapper' $last='Config' ($name=~/$last/i)
+=0
  Starting $name='OP Time Server' $last='Config' ($name=~/$last/i)=0
  Starting $name='OP Log Server' $last='Config' ($name=~/$last/i)=0
  Starting $name='OP Configuration Server' $last='Config' ($name=~/$la
+st/i)=1
Start Remaining Standard Services:
'Config'
  Starting $name='OP Data Server' $last=undef ($name=~/$last/i)=1
'Config'
Installation completed

[download]
Why doesn't ">>> In the regex" appear again when $last is undef, and how is the string 'Config' printed?

In reply to Re^4: Side effect of using undefined variable in regex by Eily
in thread Side effect of using undefined variable in regex by Anonymous Monk

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