perl -le '$x = shift; print 1 if $x and "foobar" =~ /^\Q$x/'
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update: added a first test to stop empty $x's from matching (thanks Sifmole), and a \Q in the regex as per the Perl Cookbook. Also note that in Real Life I would of course say 'foobar' and not "foobar", but that's neither here nor there.

g r i n d e r

Minor nitpick, you will test true for the empty case which is not correct.

I think that in this case the best approach uses substr:

my $matched = do {
                   my $i = length $_;
                   $_ eq substr "foobar",0,$i;
                 };
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my $matched = ($_ eq substr "foobar",0,length);
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The idea is from the CB discussion that led to <plug type="shameless">this snippet </plug>, which seems to work--I am hopeful therefore that this will also work. :-)

my @word = split //, $word_to_match;
my $match = 0;
for (0 .. $#word) {
    $match = 1 && last
        if $word_to_compare eq join //, @word[0..$_];
}
print "Matched!\n" if $match;
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$f =~ /f(o(o(b(a(r)?)?)?)?)?$/
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Wow, is that ugly. I frequently have a difficult time with nested parens in regexes, so I wrote a quick test script to use this regex to figure out where $1, $2 etc would go:

#!/usr/bin/perl -w

$string="foobar";
if ($string =~ /(f(o(o(b(a(r)?)?)?)?)?)$/) {
  print "$1\n";
  print "$2\n";
  print "$3\n";
}
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There is nothing really earth-shattering in the results:

foobar
oobar
obar
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So essentially, this will do very much what I suppose John wants to do, as the most complete match goes to $1.

Very nice, Sifmole, but I bet this tends to be a pretty slow regex.

Scott

I didn't test the speed, but I wouldn't be surprised to find out that it was slow. There are better ways to do this as are shown below; I however decided to be literal and present a regex to do what he wanted, since that was what he asked. :)

$text= "This sentence ends in foob";  # truncated from "...foobar"
$text =~ /foobar$/;
$text =~ /fooba$/;
$text =~ /foob$/;
... etc. ...
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Hopefully this isn't too obfuscated. I've tried to restrain myself here. The trunc_match function creates a regular expression for any given input string, and should handle wierd stuff to by virtue of the qr() operator.

sub trunc_match
{
        my ($what) = @_;
        my @bits;

        for (1..length $what)
        {
                push (@bits,$what);
                chop $what;
        }

        return '('.join ('|', map { quotemeta($_) } @bits).')$';
}

my $rx = trunc_match ("foobar");

$_ = "This sentence ends in foob";

if (/$rx/)
{
        print "Truncated, ends in '$1'\n";
}
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The format of the regex is something like:

(foobar|fooba|foob|foo|fo|f)$

So you get whatever you're looking for in $1, or the returned array if you're brave enough to use /g.

Update:
For some reason, I had confused qr with quotemeta, and so I am updating the code here to be more sensible in that regard. Thanks, once again, Hofmator.

That's an interesting use of qr// to escape out the individual parts, instead of using logic involving \Q..\E. I like the idea, since it avoids the confusion of escape chars in strings vs. in regex, and multiple levels of escaping. I'll be sure to remember that.