This looks like homework so I shouldn't be giving you the answer but I don't know of any better way to explain it.

use strict;
my $szBlock  = q( sub {
        if ( $x > $y )
        {
            return "X greater \n" ;
        }
        else
        {
            return "y greater \n" ;
        }    
    }
) ;
#
# $szBlock is a string containing the text of the subroutine.
#
my ($x, $y) = (100,200);
#
# Set up $x and $y (you will have to do this before the eval 
# or strict won't like it.)
# 
my $subref = eval $szBlock;
#
# if the eval worked $subref now has a reference to the 
# compiled subroutine
# 
die $@ if $@;
#
# $@ will be true if the eval failed (if there was a typo in $szBlock)
# 

#
# Now we call the subroutine by "dereferencing" the reference in $subr
+ef
# 
print &$subref;
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y greater
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