See On Interviewing and Interview Questions for how I have gone about interviewing over the years.

Further to that, I've dug out some old Perl interview questions I used to assess candidates who said they "knew" Perl (to keep them honest). That is, these questions are not difficult for a Perl expert.

• What is the CPAN?
• What does use strict; mean?
• What is the difference between $x eq $y and $x == $y?
• What is the definition of "false" in Perl?
• Generally, when should you use 'foreach' versus 'map' versus 'grep'?
• What is the difference between use File::Find and use File::Find () and require File::Find?
• What is the difference between 'my', 'local' and 'our'?
• In a regex, how do you do non-capturing parens?
• In a regex, what's the difference between "greedy" and "non greedy"?
• What is a regex assertion? Which ones have you used?

Given a list of numbers, namely:

    my @oldlist = ( 4, 7, 8 );
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write some code to add 42 to every item in this list, producing a new list. For this example data, newlist should contain the values: ( 46, 49, 50 ). Sample answer:

    my @oldlist = ( 4, 7, 8 );
    my @newlist = map($_ + 42, @oldlist);
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Given a string containing a space-separated list of names:

    my $names = "freddy fred bill jock kevin andrew kevin kevin jock";
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write some code to produce a frequency table of names, sorted descending by frequency, then ascending by name. For this data, the output should be:

    kevin     : 3
    jock      : 2
    andrew    : 1
    bill      : 1
    fred      : 1
    freddy    : 1
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Sample answer:

    my $names = "freddy fred bill jock kevin andrew kevin kevin jock";
    my %freq;
    for my $name (split ' ', $names) {
       ++$freq{$name};
    }
    for my $k (sort { $freq{$b} <=> $freq{$a} || $a cmp $b } keys %fre
+q) {
       printf "%-10s: %d\n", $k, $freq{$k};
    }
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Given an input text file and an output file as follows:

    my $infile  = 'in.tmp';
    my $outfile = 'out.tmp';
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write some code to read infile and change all occurrences of 'Peking' to 'Beijing', leaving infile unchanged and writing the changed text to a new file outfile. Sample answer:

    my $infile  = 'in.tmp';
    my $outfile = 'out.tmp';
    open(my $fhin,  '<', $infile)  or die "error: open '$infile': $!";
    open(my $fhout, '>', $outfile) or die "error: open '$outfile': $!"
+;
    while (my $line = <$fhin>) {
       $line =~ s/\bPeking\b/Beijing/g;
       print $fhout $line;
    }
    close($fhin);
    close($fhout);
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If they use s/Peking/Beijing/g contrast with s/\bPeking\b/Beijing/g and ask which they prefer.

Updated: Added \b assertions to s/Peking/Beijing/ and extra questions around regex assertions. Thanks haukex.

References Added Later

• Re^3: file modifications using file::find